[native] Set controls to visible when user backgrounds video

Summary: Currently, we pause video playback when the user backgrounds the app. However, the video controls may or may not be visible when the user returns. Now, we'll set controlsVisible to true when the app is backgrounded, so the user always sees the play button when they return to the app/can pick up where they left off.

Test Plan: Tried backgrounding and returning before/after change with controls hidden/visible before backgrounding... and it worked as expected.

Reviewers: ashoat

Reviewed By: ashoat

Subscribers: KatPo, palys-swm, Adrian

Differential Revision: https://phabricator.ashoat.com/D1110