Clarify naming on some accessibility constants and add docs

This CL continues the internal cleanup work on the
accessibility Android code. With this CL we rename
some of the static constants, and add clarifying
comments.

There are no functional changes with this CL.

Bug: 253134477
Test: No functional changes, existing tests.

Change-Id: I744b3a94fe3c3cc3b559758a95ab0b5b181155bb