Add DeepLinkManager, move startDeepLink out of StarboardBridge

[zhongqiliang]

To be reviewed after #4993 is merged.

1. Relocate startDeepLink from StarboardBridge.
2. Create a DeepLinkManager singleton to manage startDeepLink.
3. H5vccRuntimeImpl::GetInitialDeepLink() implementation in the Mojom service will now reset the deeplink to its default value after being called.

Tested on devtools. https://screenshot.googleplex.com/8DZB3z5ykQu3aTk

b/374147993

[yell0wd0g]

A few comments off the cuff. DeepLinkManager needs a unit test (which can be trivial, yes).

[yell0wd0g]

Looking good, some comments.

The JS getInitialDeepLink() behaviour is surprising (clear upon read). A comment in the IDL would be the minimum required, but perhaps we could rename the API? (Later, perhaps?)

[yell0wd0g]

LGTM with a few comment nits! Thanks for the patience during the review.