Add defer_display_events_when_frozen flag

This change adds a trunk-stable flag to control whether or not the
DisplayManagerService defers sending events to frozen processes.  This
change only adds the flag (because the flag file is a big source of
merge conflicts).

Flag: com.android.server.am.defer_display_events_when_frozen
Test: presubmit
Bug: 326315985
Change-Id: Ic0017bb09767581b543c0bf4354727efee3cf742