Adds an explicit id for visibleToInstantApps attr

Adds an explicit id for visibleToInstantApps attr.

This fix is to enable Instant Apps to use the gcore that will ship
with Android O. Gcore is not building against  the O SDK, so to
enable them to use the visibleToInstantApps attribute, we are going
to provide them with a modified public.xml to use in the api 25
SDK.

Test: Ensured coretests / FontResourcesParserTest continues to pass.

Bug: 36036242
Change-Id: I5a9b878cdbffe9323b299e8dbe82c599e33a6834