Represent hidden classes' subclass definitions in partial signature files

Partial signature files should represent all delta between the API
surfaces. This include the definition of subclasses that are modified
when it extends a class that is hidden for some API surfaces.

This change adds the currently missing such information to the signature
files.

Bug: 295032200
Test: m update-api
Merged-In: I0f9e0720b19ed8fec6d7fa77be97d565d1dd1f16
Change-Id: I757f210b0e38b0a2a629fa1f37eee629bcfca804