OMS: Make updates to category not reset the overlay's state

- Also change how default overlays are applied. Apply them if no other
overlay is enabled for the same category as the default overlay.

Bug: 73740686
Test: manual
Test: adb shell rm /data/system/overlays.xml && adb reboot #verify that colors are blue
Test: adb pull /data/system/overlays.xml \
          && <edit overlay to remove categories> \
          && adb push overlays.xml /data/system/overlays.xml \
          && adb reboot # verify that colors are blue

Change-Id: If42afd287703e2e159d0087e477c19409f724124