Fix RemoteTargetGluer crash

- Updated DesktopVisibilityController to track the numer of visible
  freeform windows, and use it to initialize RemoteTargetGluer.
- Updated IDesktopTaskListener to observe visible freeform windows count
- Added resize remoteTargetHandles logic in RemoteTargetGLuer to update
  handles size with targets.apps.size on assign.
- This is a workaround and should be removed when RemoteTargetGLuer
  intialisation logic is refactored.

Bug: 288121021
Flag: None
Test: atest DesktopModeTaskRepositoryTest DesktopTasksControllerTest
Change-Id: Ibcb12ef4c1a46f92d6a15e01e45ffcf2d53c6fa0