Added some CTS test for picture-in-picture mode.

- Host side test to make sure activity is put in the
pinned stack when it is piped
- Device side test to make sure the Activity#enterPictureInPictureMode
works as expected depending on the activity attribute and device support.

Bug: 25580820
Bug: 19225708
Change-Id: I89d7ebaee6318c5e00f7f47e0a5e16ba66beb132
10 files changed