ExternalDisplayStats on content mode changes

ExternalDisplayStatsService keeps track of the extended or mirror
setting based on display count. When it logs an event with display
count = 1 then metric will count it as content mode change.

In addition to this, right now there is only global settings toggle to
switch between extended and mirror globally. All displays are assumed to be either mirroring or extended.

Whenever display is getting disabled through disableConnectedDisplay API , this event is currently not logged. From metrics perspective display will stay in the content mode chosen in settings, and not in disabled state.

If the display is just connected and not yet Enabled by the user through the "Mirror/Dismiss dialog" - display stays in "Connected" state, not "mirroring" from metrics perspective.

With this CL, all External Displays get auto-enabled if the user switches "mirror"/"Extended" settings toggle. In addition to this, stats service now logs the event of switching between mirror/extended.

Bug: 400742534
Bug: 400384229
Test: atest ExternalDisplayStatsServiceTest ExternalDisplayPolicy
DisplayManagerServiceTest
Flag: com.android.server.display.feature.flags.enable_display_content_mode_management
Change-Id: I3c11f39f9028306443343a4b5f8c6b72f1e1e667