Cumulative wakelock time per uid

Currently, the time for each partial wakelock was tracked. If one
wants the total time that a uid held partial wakelocks (over all of its
wakelocks), they could sum over all the uid's partial wakelock
totalTimes, but this would underestimate the value because totalTimes
are pooled amongst all uids. Alternatively, they could sum over all the
uid's partial wakelock totalDurations, but this would overestimate the
value because totalDurations are not pooled within the uid (so there
will be double-counting if two wakelocks are held simultaneously). This
cl adds a new timer that specifically tracks the actual total time that
the uid spent holding wakelocks, and also provides the ability to see
how much time the uid was in the background when doing so.

Bug: 38198272
Test: runtest -x frameworks/base/core/tests/coretests/src/com/android/internal/os/BatteryStatsTests.java

Change-Id: I20ea3546338c44ffdf06859bc840d9059fb18321