C2SoftAvcEnc: Allocate output buffer as per clip's dimensions

Instead of allocating a fixed size output buffer, allocate it as per
width and height of the input being encoded. To ensure for very low
resolutions, this size doesn't become too small, use a minimum size of
512K Bytes.

Bug: 144928581
Test: h264 related tests in android.media.cts.VideoEncoderTest
Change-Id: I01c1aa03456043824354005f1708f07b3268d97e