Support Picture-in-Picture on Chrome OS

enterPictureInPictureMode() must be called in onUserLeaveHint() to
support Picture-in-Picture in a multi-window environment such as
Chrome OS.

The current implemententation (calling the API in onPause()) causes
the following issues on Chrome OS:

- VLC enters PiP without being occluded, but just with another
  window activated. In a multi-window environment, we can't assume
  onPause() means the window gets hidden. As only one window can
  be resumed at the same time, it's possible that a window is
  fully visible, but paused.
- When some ChromeOS-specific system UI gets shown (e.g. Alt-Tab
  window cycler, Overview Mode), VLC enters PiP unexpectedly.

This patch moves switchToPopup() from onPause() to onUserLeaveHint()
to prevent those issues.