Track Partial wakelock time in background

Changed partial wakelock time to be a DualTimer so that it can also
track the time spent while app was in background.

Bug: 62134255
Test: cts-tradefed run cts-dev -m CtsDumpsysHostTestCases -t android.dumpsys.cts.BatteryStatsDumpsysTest
Change-Id: I85cca468ac126ee83a3600800bcfa75c9fc3012f