Don't handle the system UI visibility at server side

The system UI visibility has been deprecated. Except disable flags, it
should be converted to the new APIs before being sent to WMS. This CL
renames mSystemUiVisibilitiy to mDisableFLags in WindowState.

Bug: 149813814
Test: atest TaskSnapshotControllerTest TaskSnapshotPersisterLoaderTest
Change-Id: I6730ba664359656ee814c7bc959844d6ed4aaeb9