power.stats: Add more informative default implementation

The original default implementation for power.stats HAL did not have a
good example implementation of the power statistics related APIs. Adding
a new default implementation that gives a more informative example

Bug: 122267057
Test: run vts -m VtsHalPowerStatsV1_0Target
Test: adb shell "lshal debug android.hardware.power.stats@1.0::IPowerStats/default"
Observed the following output
========== PowerStats HAL 1.0 state residencies ==========
          Entity            State         Total time     Total entries   Last entry timestamp
   DefaultEntity           Active               1 ms                 2               3 ms
   DefaultEntity            Sleep               4 ms                 5               6 ms
========== End of PowerStats HAL 1.0 state residencies ==========

Change-Id: Ida0951c267f609b16bb6406da150ed2e504ced9a