Add -Werr to `apksigner verify` when Autograph supports it

[severinrudie]

Vision statement / What / Requirements

tools/sign_release.sh currently runs apksigner, but does not crash on warnings. This is because Autograph uses apksigner v1 instead of v2, meaning that it does not sign files found in META-INF. These files cannot be cleaned up, as many are important to AndroidX.

Once Autograph has updated to using apksigner v2, we should add the -Werr (crash on warning) option back to our call to apksigner verify.