WM: Make AccessibilityController persistent

Keep the AccessibilityController alive so we can keep track of
focus changes from input. This will allow Accessibility service
to query the focus window and get the focused embedded window
as well.

Bug: b/188792659
Test: presubmit

Change-Id: I5741ed373938963da129c410c5226dff5a84ce30
Merged-In: I5741ed373938963da129c410c5226dff5a84ce30