Reduce wake lock noise in battery history.

When the work source of a wake lock was changed, this would
cause the old wake lock to be released in battery stats before
the new one was acquired (the power manager would correctly
keep holding the associated wake lock).  This resulted in a
pointless entry in the battery history showing the last wake
lock being released and a new one acquired.

This change adds a new path in to battery stats to report
when a wake lock has changed, allowing it to acquire the
new wake locks first before the old ones, so it can't drop
down to zero wake locks.  This also provides better timing
information, as the same current time can be used for both
operations.

In addition, added a new kind of history entry for the
current time, so you can tell when in actual world clock
time the battery data is happening.

Change-Id: Ibbf2eed83bb93f31f60267889b7bc5b9e71e355f