MPEG4Writer: Handle writing multiple NAL units in a video sample

Encoder may prepend extension NALs (eg: AVC SVC extension NAL
for temporal layer info) to the VCL NAL.
Look out for non-VCL NALUs preceeding the slice NALs and write
them out separately.
Search for a limited size since such NALs are of small size.

Bug: 27596987
Change-Id: I434c5252098faca99c0ee18e6bc5f43a67718eba