libstagefright: Unbreak live effects

Change I1e31f2db521834a068e11d48e88547044b0c906f broke live effects
for (probably all) non-Samsung msm8660 devices. Use platform-based
logic to choose the correct memory pool (SMI for 8660 and MM for
everyone else), and add a build-time switch for devices that always
want MM regardless of platform (BOARD_CAMERA_USE_MM_HEAP)

Change-Id: I103946c40548f319bb1446f75a6a539aa3eb51b8