Camera: Fix high speed recording failure due to prepare()

In case application sends 2 consecutive setRepeatingBurst, the first
request may be preparing the video stream, whiel the second request
fails the isPreparing check.

Fix this by distinguishing between internal and external prepare. For
internal prepare, allow new requests to come through to the request
thread.

Bug: 114422231
Bug: 109830370
Test: Camera CTS
Change-Id: I55a7271e3924f2cb8cf9c452e934b070a82bc4ca