Renamed a few methods in ActivityRecord (42/n).

This is in preparation for having the ActivityRecord class
extend AppWindowToken where name distinction will be important.
There is no functional change in this CL.

ActivityRecord.realActivity -> mActivityComponent
ActivityRecord.userId -> mUserId
ActivityRecord.getTask() -> getTaskRecord()
ActivityRecord.getStack() -> getActivityStack()
ActivityRecord.service -> mAtmService
WindowContainer.mService -> mWmService

Bug: 80414790
Test: Existing tests pass.
Change-Id: Ie5788db7f2e823c7d5b64c7ce6eaade1c19c4dcd