Fix onDisplayAreaInfoChanged called before onDisplayAreaAppeared

1. In Core, add a flag to track appeared sent.
2. In Shell, make sure the #registerOrganizer is called on the main
   executor.

Fix: 302796032
Bug: 297469287
Test: atest WmTests:DisplayAreaTest
Test: atest WMShellUnitTests:ShellTransitionTests
Change-Id: Ia3142c281479cdf46945d301b96f9f2cd1682166