libstagefright: Disable smoothstreaming for secure playback

Disable Smoothstreaming if the property "mm.disable.sec_smoothstreaming"
is set.

CRs-Fixed: 664732

Change-Id: I36da2e3bf73c802166e62ea3bfadeac523663b77
Merge "libstagefright: Disable smoothstreaming for secure playback" into du44