psh_utils: Clean up log printing

1) Allow prefix for dumpsys indentation.
2) Suppress printing empty information.
3) Indicate battery stats values are net (i.e.
   system power loss + charging cable).

Flag: com.android.media.audioserver.power_stats
Test: atest powerstats_collector_tests
Bug: 350114693
Change-Id: I72ad845430bacd39e8dd64e0c055c30bca85bc6c