Updating PiP API and documentation.

- Renaming PictureInPictureArgs to PictureInPictureParams, making it
  immutable and built with a builder class.  In addition, making the
  aspect ratio rational.
- Moving the check to get max number of actions to the activity to have
  more flexibility going forward
- Also ensures that enterPictureInPictureMode() returns true if the
  activity is already in PiP mode.
- Will follow up with CL to remove PictureInPictureArgs and related methods
  once this CL drops

Bug: 35765211
Bug: 37638323
Bug: 37637423
Bug: 37636674
Bug: 36342780
Test: android.server.cts.ActivityManagerPinnedStackTests
Test: android.app.cts.PictureInPictureParamsBuilderTest

Change-Id: I54a487d55a59c9ce8ac7bd6c80c61209c8b26224
Signed-off-by: Winson Chung <winsonc@google.com>