Set isInPictureInPictureMode earlier

Per https://developer.android.com/guide/topics/ui/picture-in-picture
After app requests enterPictureInPictureMode and receives onPause
callback, it will continue playback if isInPictureInPictureMode is true.

However, with ag/11273366, isInPictureInPictureMode will now return true
after the new configuration is dispatched to app, which happens after onPause.
This may cause app, following the guidance, to cease playback in PiP mode.

Fixes this by setting the internal mIsInPictureInPictureMode earlier
right in enterPictureInPictureMode

Video: http://go/recall/-/aaaaaabFQoRHlzixHdtY/fVRqG7UWoKkQQhFxPkzcUt
Bug: 156924033
Test: manually enter PiP from Twitch
Change-Id: I8e0865076fcb756cfa5db39901f460ab5ad69b99