libstagefright: Allow multi-byte CSD sizes for non-m4v clips

As per ISO/IEC 14496-15, there is no explicit limit on the size of
codec-specific-data for AVC clips. Only mpeg4 video requires that
CSD size must fit within a single byte. Make the assertion enforcing
this condition specific to m4v content.

CRs-Fixed: 724526
Change-Id: Id006030ac65d5ec0bf285c040e4c536039f9489b