Sometimes viewfinder is black when re-launching camera app.

[KasperLau]

$ system-image-cli -i
current build number: 283
device name: krillin
channel: ubuntu-touch/rc-proposed/bq-aquaris.en
last update: 2016-03-17 03:48:27
version version: 283
version ubuntu: 20160316
version device: 20160108-efc96d8
version custom: 20160111-926-36--vivid

Steps:
1. Launch Camera and swtich to record mode.
2. Tap "Record" button.
3. Swipe right to reveal Launcher and tap Ubuntu button to go to Today scope.
4. Swipe right to reveal Launcher again then tap Camera button.

Actual result:
Sometimes viewfinder is black when re-launching camera app.

Expected result:
Camera works well.