Hide release-specific BaseFragmentActivity APIs

These classes should not have been public to begin with. Hiding them now
as a cleanup step.
A different variation of this CL went into the N release; this CL is
being done to account for the reorganized support library structure.

Issue #28775596 API Review: BaseFragmentActivityJB

Change-Id: Id88e6b95199bf7890e6864129087ef1ccf5a44ff
4 files changed