modify quick settings to adopt the accessibility state api

Seperated out the state part from contentDescription. Also changed the
tile on/off states from mocking switch to unavailable/on/off state
description based on State.state. During testing, I removed event
merging (including source node merging) for state and content change
events. Use event text for state description changed part will be in a
seperate CL.

Test: tested on the quick settings tiles for focusing and state change
evnets. With the event merging removed, the state change is announced
when the focus keeps on the tile. Before my change, content change is
not announced when the focus keeps on the tile due to source node change
in event merging.

Fix: 148473520

Change-Id: I5b6aee6f0982cf4e0a3962fbe15239314f3a2de4