Use dark theme for player controls.

Currently uses the wrong accent colour since there’s no dark variant of
the Ach so! theme yet.
Fix some action mode quirks.

- Action mode would lose the selection but stay active after onPause().
- Selecting items using a long press would not work after the initial
one.
Remove OverlayCheckableLayout.

It's easier and more flexible to implement manually.
Use a more descriptive name for the data observer.
Use events when uploading and chain uploaders.
Keep Otto annotations.
Merge branch 'improvement/LL-831' into review

Conflicts:
	app/src/main/java/fi/aalto/legroup/achso/adapter/VideoGridAdapter.java
	app/src/main/res/layout/partial_playback_controls.xml