Support chained attribution for alarm related WorkSources.

.. those passed down via the AlarmManager.set() variant that takes
a WorkSource. This required a minor re-arrangement of code in
the ActivityManager. We now treat WorkSources as opaque in the
AlarmManager and simply push them down to the AM (and eventually
to BatteryStats) where they are picked apart.

Test: BatteryStatsNoteTest, AlarmManagerTest
Bug: 62390666
Change-Id: I118f1a1d16aafa41b4f401f1a6a3ba4d2d5eca8f