Block a11y-changing key events from a11y services

If the volume keys are being pressed to enable or disable
an accessibility service, the key events should not be
dispatched to accessibility services.

Bug: 62653966
Test: A11y CTS and unit tests. Adding a set of unit tests for
the expanded KeyboardInterceptor. Also verified that the
accessibility shortcut now works as designed with TalkBack.

Change-Id: Iaea58a5bfe6748d4b9a033f5b957e78298881c40