Separate metadata provider from apex contents provider

The platform_compat_config_singleton only needs access to the metadata
so this separates that method into its own interface, distinct from the
one needed by the apex. This also adds a test to ensure that the merging
still works.

Bug: 182402754
Test: m nothing
Change-Id: I5212239786810e5fc5eb99831b1122db93d1329f