Declare official platform slice

Create the notion of an official platform slice.
This includes:
- Adding a second authority to the provider
- tagging slices in xml with a platform slice flag
- Including authority in the getUri method

Bug:73359139
Test: robotests
Change-Id: I5382be138a262dbc5a8324c34aab131c5d0d5516
Merged-In: I581ee6dfcdf935f452a15e89e5d055e375ff1877