CMFileManager: Fix back press events

Use onBackPressed in stead of onKeyUp for back press events

Repro:
- Navigate to audio file
- Open audio file with small player (e.g. Play Music)
- Press back
- Observe: audio player closes and CMFM navigates up one directory

Change-Id: Ia7440c45241ec957b2405b932525235c92b9211c