Move PiP state check exception to the client side

- When calling enterPictureInPictureMode(), the state of the activity in
  the client may be out of sync with the state of the activity in the
  system, causing an exception to be thrown erroneously. Instead, fail
  silently in the system if this occurs, and throw the exception in the
  client when it attempts to enter PiP from an invalid state.

Bug: 63753007
Test: android.server.cts.ActivityManagerPinnedStackTests

Change-Id: Ia99cc086805edc31f997d4325f7a5ccd7c85a77e