Fix lock contention: Call into power manager service from handler

Make sure to not hold the wm lock when calling into power manager
service, because PWM will acquire a lock that might be contended.

Test: Make sure user activity timeout is still respected on
Keyguard
Test: Have activity with screenBrightness=1.0, make sure screen
is fully bright when opened

Bug: 37888898
Bug: 36631902
Change-Id: I4b5433dbaf8aa151465ae32232d3b3b8597715df