Add a Impl_only_libs prop for sdk_library

Similar to Stubs_only_libs, this allows being a bit more specific
about what libs compile against what. In my usecase, it allows
removing a platform dependency from the appsearch stubs, which allows
the platform to depends on the stubs.

Bug: 169304493
Test: m
Change-Id: Ie997462819cd5266a761b2d415e81806c7877967