Fix usage of java.import.generatesMetadataFilesAtProjectRoot

lsp-java used to treat this as a configuration property of jdtls.
However it is not a standard lsp configuration property. Instead it
must be passed as a Java System Property on the command line invoking
jdtls. See
[JLFsUtils.java](https://github.com/eclipse-jdtls/eclipse.jdt.ls/blob/master/org.eclipse.jdt.ls.filesystem/src/org/eclipse/jdt/ls/core/internal/filesystem/JLSFsUtils.java#L192-L197)
for how this property is read. Also see
[eclipse-jdtls/eclipse.jdt.ls#1900](eclipse-jdtls/eclipse.jdt.ls#1900 (comment))
for an explanation of why this is a system property and not a regular
LSP configuration property.

Use a regular `defcustom` instead of using `lsp-defcustom` for
declaring lsp-java-import-generates-metadata-files-at-project-root.
Use this defcustom variable in `lsp-java--ls-command` to pass an
appropriate value for
`java.import.generatesMetadataFilesAtProjectRoot`.

With this change importing a Gradle-based java project into LSP will
nolonger create `.classpath`, `.settings`, and `.project` entries at
the root of the project.

Author: crabbkw

lsp-java.el

@@ -128,6 +128,12 @@ deduplication with the G1 Garbage collector"
   :risky t
   :type '(repeat string))
 
+(defcustom lsp-java-import-generates-metadata-files-at-project-root nil
+  "Specify whether the project metadata files(.project, .classpath,
+.factorypath, .settings/) will be generated at the project root."
+  :risky t
+  :type 'boolean)
+
 (lsp-defcustom lsp-java-errors-incomplete-classpath-severity "warning"
   "Specifies the severity of the message when the classpath is
 incomplete for a Java file"
@@ -688,7 +694,11 @@ FULL specify whether full or incremental build will be performed."
                                lsp-java-server-config-dir
                              (lsp-file-local-name (lsp-java--locate-server-config))))
             (java-9-args (when (lsp-java--java-9-plus-p)
-                           lsp-java-9-args)))
+                           lsp-java-9-args))
+            (generate-metadata-property (if lsp-java-import-generates-metadata-files-at-project-root
+                                            "-Djava.import.generatesMetadataFilesAtProjectRoot=true"
+                                          "-Djava.import.generatesMetadataFilesAtProjectRoot=false"
+                                          )))
         (lsp-java--ensure-dir lsp-java-workspace-dir)
         `(,lsp-java-java-path
           "-Declipse.application=org.eclipse.jdt.ls.core.id1"
@@ -697,6 +707,7 @@ FULL specify whether full or incremental build will be performed."
           "-Dlog.protocol=true"
           "-Dlog.level=ALL"
           ,@lsp-java-vmargs
+          ,generate-metadata-property
           "-jar"
           ,server-jar
           "-configuration"
@@ -1582,14 +1593,6 @@ data. Beyond this limit, cached workspace data may be removed."
   :type '(repeat nil)
   :lsp-path "java.configuration.workspaceCacheLimit")
 
-(lsp-defcustom lsp-java-import-generates-metadata-files-at-project-root nil
-  "Specify whether the project metadata files(.project, .classpath,
-.factorypath, .settings/) will be generated at the project root.
-Click [HERE](command:_java.metadataFilesGeneration) to learn how
-to change the setting to make it take effect."
-  :type 'boolean
-  :lsp-path "java.import.generatesMetadataFilesAtProjectRoot")
-
 (lsp-defcustom lsp-java-project-output-path ""
   "A relative path to the workspace where stores the compiled
 output. `Only` effective in the `WORKSPACE` scope. The setting
@@ -1841,14 +1844,6 @@ Beyond this limit, cached workspace data may be removed."
   :type '(repeat nil)
   :lsp-path "java.configuration.workspaceCacheLimit")
 
-(lsp-defcustom lsp-java-import-generates-metadata-files-at-project-root nil
-  "Specify whether the project metadata files(.project, .classpath,
-.factorypath, .settings/) will be generated at the project root.
-Click [HERE](command:_java.metadataFilesGeneration) to learn how to change the
-setting to make it take effect."
-  :type 'boolean
-  :lsp-path "java.import.generatesMetadataFilesAtProjectRoot")
-
 (lsp-defcustom lsp-java-project-output-path ""
   "A relative path to the workspace where stores the compiled output.
 `Only` effective in the `WORKSPACE` scope.  The setting will `NOT` affect