hiddenapi: Handle visibility through inheritance

Framework uses a pattern in which a public method is defined in
a package-private parent class, but exposed outside of package
via a public child class. Improve the definition of stub member
visibility in `hiddenapi` to understand this pattern.

Bug: 76424618
Test: m appcompat, SpannableStringInternal.length() public
Change-Id: Ie9c1653142a4991d6de5460be5abd074aa03b0a0