Remove defer_display_events_when_frozen flag

The flag is removed.  Conditionals that relied on the flag are
collapsed.  Code that is no longer used is removed.

Flag: EXEMPT removing com.android.server.am.defer_display_events_when_frozen
Bug: 423927562
Test: atest
 * CtsDisplayTestCases
 * DisplayEventTest
 * DisplayServiceTests

Change-Id: I23b604f8fff3d0e976421ea0f4bff01686073115