Update standby bucket for uninteracted foreground services.

This fixes a bug where foreground services that were started, but
never interacted with, were not transitioning to the Active bucket
after 30mins - a defined interval.

Also created unit tests for #maybeUpdateUsageStats().

Test steps: (adb shell commands are generic)
1) Install a test app which has a foreground service
   * can install test app provided in bug via adb install
2) Set the bucket of the test app to Rare:
   $ adb shell am set-stanby-bucket com.packagename rare
3) Launch the foreground service
   $ adb shell am broadcast -a com.packagename.intentfilter -n com.packagename/.MyReceiver
4) Wait for approximately 30mins
5) Observe the app bucket - should be Active (10)
   $ adb shell am get-standby-bucket com.packagename

Bug: 116189835
Test: atest OomAdjusterTests
Test: manual (steps listed above)
Change-Id: I6949466297cccdc349428c5f6172d65bd9d77a53