Move input focus for a11y actions with WM instead of ActivityTaskManager

Moving input focus is required to show IME after calling
ANI#performAction(ACTION_CLICK) on an embedded text view's node.

ActivityTaskManager#setFocusedActivity does not transfer
window input focus to embedded views.

Bug: 278846790
Test: new test in AccessibilityEmbeddedHierarchyTest
Test: use TalkBack to click on embedded (same & cross-process) edit
      texts, observe IME opens and `dumpsys input` shows the embedded
      "window" as the input FocusedWindow.
Test: same as above, but in split screen mode with two apps each with
      their own edit texts.
Change-Id: I7bd7783cd3426b547ddd2d0e71ad5b38281198c3