Removed obsolete a11y support for ClickableSpan

Since we now support ClickableSpan accessibility natively,
we no longer need elaborate workaround for them. The helper
had its own issues, since it meant that some TextViews were
presented completely differently to accessibility than
others (thus the bug that led me here.)

Bug: 37363351
Test: Verified that Select to Speak can now highlight the
footer text in Settings properly. There is already a CTS
test for the ClickableSpan support.

Change-Id: I5dd4fb745c2c994099ca6c3332beff9a58227359