Add accessibility state API in platform

The detailed proposal is here:
https://docs.google.com/document/d/1zeR67kiDmLUJ2CENXjJ2-O3REzhC7MtyYGjV_yMj2g0/edit#heading=h.o3tvsnccns69

Test: Tested with the "bluetooth" tile in quick settings. First call
setStateDescription() in
handleStateChanged(QSTile.State state), then add log to dump the
accessibility info string in
onInitializeAccessibilityNodeInfo(). With talkback enabled, checked that
the logs have the correct state description field.
Bug: 140882622

Change-Id: I658f7972fec2a4c8abeb9470d2bda2104db053df