Remove direct WM lock from WPC#onTopProcChanged

Currently it is only used for VR mode. If it is not enabled, nothing
needs to be done. And if the proc state is change while VR is enabled,
post a message to update the state to avoid nested lock.

Also a minor cleanup for non-null WindowProcessListener.

Bug: 159104503
Test: WindowProcessControllerTests
Change-Id: If30ab2336dfdb56e3871845d65fe298aa696633d