a11y: Remove TalkBack keyboard shortcut handling from PhoneWindowManager

With key gesture handler APIs, we can handle shortcuts in A11y
directly, this can allow components to react appropriately.
e.g. Use displayId passed in key gesture event to show UI on
correct display instead of routing that information from PWM to
A11y service.

Attaching the original cimmit that added TalkBack keyboard shortcut for
reference: I42d61f620b7b15e0e4d215d6b99b090b9862b15d.

Note that TalkbackShortcutController.java is used by wear via
`TRIPLE_PRESS` so we'll keep it. Further refactor can be done in
following CL in case needed.

Change-Id: I1981de4e30a9b4fda7f13056d91b3be7e80c10ef
Bug: b/413279228
Test: atest KeyGestureEventTests
Flag: EXEMPT refactor