Fix an issue with apps EXEMPTED after OTA

After an OTA, all system apps were being
pushed into EXEMPTED for 4 hours because
of a race with the boot phase resulting in
it appearing as if app standby was disabled.

Another bug in updating the state out of EXEMPTED
was preventing checkIdleStates() from fixing
the issue soon after.

Bug: 72835804
Test: Manual:
        Manually delete /data/system/usagestats/version
        Reboot
        Verify that correct apps are EXEMPTED
      Automated:
        atest FrameworksServicesTests:AppStandbyControllerTests
Change-Id: Ib53f0c45e5c2e2456442f6782ad5ca9b9d0c3d72