Improve fullscreen, orientation and PiP handling

[ThreeFive-O]

Fixes #174.

See: https://developer.android.com/training/system-ui/navigation

When the user reopens the activity, onCreate() won't get called, so the system bars will remain visible. If you want system UI changes to persist as the user navigates in and out of your activity, set UI flags in onResume() or onWindowFocusChanged().

[Maxr1998]

Good catch, thanks for the fix. Could you move the onResume function below onStart please? That order makes more sense imo considering the activity lifecycle.

[ThreeFive-O]

@Maxr1998 Thank you for the feedback. Fixed the feedback and the code comment which was outdated.

One thing I noticed is that onResume doesn't fully fix the case when returning (maximizing) from Picture in Picture (PiP) mode. In most cases the navigation bar is still visible, but can be hidden by tapping the player window. In rare cases the navigation bar hides when returning from PiP.
Returning from PiP calls onResume, but there must be some other problem involved.
I'm on Android Pie on a Samsung phone.

Any idea?

[Maxr1998]

I've observed that as well when first implementing PiP, but never found a solution/fix for the issue. I'm thinking it's a bug in the Android framework or ExoPlayer library, but I can't really tell.

YouTube opens the video in portrait mode when returning from PiP, so the bug doesn't apply there. Maybe we should do the same to cover it, it also makes sense since you mostly watch through PiP while in portrait.

[ThreeFive-O]

@Maxr1998 It took me a while to figure out the fullscreen logic correctly, but I guess with the latest commit I addressed all issues.

The behavior of the latest commit is as follows:

• When starting the video playback we request landscape orientation. The configuration change handler will take care about fullscreen mode.
• When playback is stopped and app is paused (because of an app switch) the onResume handler will restore the last viewing mode (fullscreen when it was fullscreen; normal mode when it was normal mode)
• When switching to PiP mode the configuration changes for enabling/disabling fullscreen mode are not handled. This fixes the rogue orientation changes in the configuration change handle. (Wild guess: Looks like delayed configuration change events handled after leaving PiP mode which arrive in unsorted manner and therefore can cause false reporting of orientation changes)

Please give it a try.

[Maxr1998]

Thanks for the other set of changes, looks good to me! Just some small formatting nitpicks to fix, then it's ready to merge!

[ThreeFive-O]

Thank you @Maxr1998 and @nielsvanvelzen for your code reviews and improvements.

[Maxr1998]

Added another small fix (the isPictureInPictureMode call would have crashed below N) and rebased/squashen all commits onto the lastest master.