video: fix incoming video for iOS

During incoming calls to iOS, video input failed to open sometimes.
It happens for two reasons:
1. Stopped by interrupt callback.
2. Keyframe callback was set two late.

This patch:
1. Completes decoder configuration after video loop start running.
2. Sets keyframe callback for video session before starting.

Change-Id: I00e1ace06a78ab2ec2fb3ce6fe6a5991c6593318