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
Change-Id: Ife61f2abd673db93d4af6ee9affd4119cc17f7f2
Merged-In: Ia7835d04d988127d061d3a454b8391294346a915