Add API to track usage time of apps.

This adds a new ActivityOption for the caller to ask the
system to track the time the user is in the app it launches,
delivering the result when they are done.

The time interval tracked is from when the app launches the
activity until the user leaves that app's flow.  They are
considered to stay in the flow as long as new activities
are being launched or returned to from the original flow,
even if they cross package or task boundaries.  For example,
if the originator starts an activity to view an image, and
while there the user selects to share, which launches gmail
in a new task, and they complete the share, the time during
that entire operation will be included.

The user is considered to complete the operation once they
switch to another activity that is not part of the tracked
flow.  For example, use the notification shade, launcher, or
recents to launch or switch to another app.  Simply going
in to these navigation elements does not break the flow
(although the launcher and recents stops time tracking of
the session), it is the act of going somewhere else that
completes the tracking.

The data is delivered to the app through a PendingIntent,
which includes the total time the app was in the flow along
with a time break-down by app package.

Change-Id: If1cf8892d422c52ec5042eba0e15a8e7e8f83abf