Camera3Device: Support batch requests

Refactor request threadLoop to three parts: waiting for next
batch of requests, preparing HAL requests and output buffers for
next batch of requests, and submitting the batch of requests to
HAL.

Set the batch size to the size of the request list if it's a video
recording request in a high speed video configuration.

Add a flush lock so that HAL's flush() won't be called while
submitting a batch of requests.

Bug: 23360060
Change-Id: Icd395b1f955a9b336eec6fa5aff6b17741ce08c7