statsd: Migrate statsd SubsystemSleepState to use power.stats HAL

statsd pulled event SubsystemSleepState will now use power.stats HAL if it
is available and supported. Otherwise it will fall back to the original
implementation of using the Power HAL.

Bug: 120551881
Test: adb shell cmd stats pull-source 10005
Test: Removed IPowerStats from manifest and ran above command. Verified
that it fell back to using IPower HAL
Test: Killed HAL services, waited for them to start back up, and
verified that statsd was still able to pull the SubsystemSleepState.
Change-Id: I852aa3386b2f2e77865ea6b4ff4cdc6e48cee77c