SoftAVCEncoder: Automatically select the right level for the chosen resolution

Previously a certain level was hardcoded, and unless the caller
(at the MediaCodec API level) explicitly specified a desired level
to use, the SW AVC failed to encode resolutions that require a
higher level.

This makes the 720p tests in EncodeDecodeTest pass with this
encoder.

Change-Id: Id0d47da5cdf6beb5dc17aae08c258b3490ce52a3