Add new api Activity.getStartInitiatedTime.

This api will return the timestamp at which this activity start was last
initiated by the system. Implementation is wip.

Bug: 9058261
Test: cts-tradefed run singleCommand cts-dev -m CtsAppTestCases -t \
      android.app.cts.ActivityStartTimeTest

Change-Id: I396458ecefbb09108f414b95f9c0beb6d609a4e1