Fix NotificationShade toggling FLAG_NOT_FOCUSABLE

This fixes some issues caused by the NotificationShade becoming
focusable momentarily (by temporarily toggling off FLAG_NOT_FOCUSABLE
on their window) when locking the screen and animating in AOD.

This is essentially a revert of [1], which meant to make the window
focusable to block touches on the app under the NotificationShade
window. The problem with this approach is that focusability does not
impact where the touch events are actually dispatched, so it didn't fix
the original issue, but does create issues with the IME.

  [1]: I81843d4901f6f3bb0bb00f3d8eaa713da494b0fe

Test: FlickerTestsIme2:com.android.server.wm.flicker.ime.ShowImeOnUnlockScreenTest
Bug: 300446848
Flag: NONE
Change-Id: Ief3a46664ef87e4c23c732189867eda63e9e5688