Camera: Use a single handler thread for device state callbacks

Use a single handler thread within the same camera client process
for processing device state changes.
This should avoid cases where clients continuously instantiate and
cache CameraManager instances until the process FD limit is reached.

Bug: 270551856
Test: atest -c -d
cts/tests/tests/virtualdevice/src/android/virtualdevice/cts/VirtualDeviceManagerBasicTest.java#getVirtualDevices_returnsAllVirtualDevices
--iterations=10
and ensure that the handler thread for device state callbacks remains
the same.

Change-Id: Ia659075947ab6c3f2bf56119b06f6397953dbd5a