Clean up PowerManager API.

Mostly just moved the code around and improved the docs.

Fixed a minor inefficiency in WakeLock.setWorkSource() where
we would tell the power manager that the work source was changed
even when the old work source and new work source were both null.

Fixed a bug in WakeLock.setWorkSource() where we would not
notify the power manager that the work source was changed if
there was no work source previously specified.

Added synchronized to WakeLock.setReferenceCounted.

Added a checked in WakeLock.acquireLocked() and WakeLock.release()
to check whether the wake lock is actually not held / held
before performing the corresponding operation.  This change avoids
making redundant calls into the power manager service in the
case where the wake lock is not reference counted and acquire()
or release() have been called multiple times.

Made the PowerManager and WakeLock classes final.  They are not
directly instantiable by applications so this change does not
break the API.

Removed a little dead code (one private constructor and an
unused constant).

Change-Id: I4e10cf893506115938a35756136c101256dccf30