Add <staging-public-group> tags to T

To prevent apps from having to respin atomically with SDK finalization,
<staging-public-group> must be created with type ids that do not
overlap the public resource id namespace.

Test: builds, aapt2_tests
Merged-In: Ia7835d04d988127d061d3a454b8391294346a915
Change-Id: Icf5c56e9bc7dea28d018acf7f9ae5c9d4e3eba31