Improve tracking of screen on reasons.

- New screen on app op to record the last time each app has
  caused the screen to be turned on.
- New battery stats event that tells us the reason the screen
  has been asked to turn on.
- Propagate out power manager API to specify the reason a caller
  is asking to have the screen turned on.

Note that currently the window flag to turn the screen on bypasses
much of this because it is being handled in the window manager by
just directly telling the power manager to turn the screen on.  To
make this better we need a new API where it can specify who it is
calling the API for.

Change-Id: I667e56cb1f80508d054da004db667efbcc22e971