Remove WM lock from WPC#computeOomAdjFromActivities

Since the result of computeOomAdjFromActivities depends on task
layer rank and activity state. It can be calculated once the states
are changed before oom-adjustment calculation. So when activity
manager asks the result from any reason, it can get the computed
states without entering heavy WM lock.

Also remove lock in WPC#hasVisibleActivities because its returned
result is also computed in computeOomAdjFromActivities.

Bug: 159104503
Test: WindowProcessControllerTests#testComputeOomAdjFromActivities
Change-Id: I797c81a5ebe447802cc02ac37736a0464108836f