Eliminate special battery calls by WindowManager

Previous to this change the WindowManager was notifying the
BatteryDtatsService about windows that keep the screen on. WM used a
custom WakeLock tag to indicate to PowerManagerService that it had
already notified the BatteryStatsService.

This change eliminates WindowManager notifying the BatteryStatsService
and lets PowerManagerService do the job.

Fixes bug 7030326.

Change-Id: I666dc6ef8f094b8d3d109fea6876be058e057b4f