libstagefright: Add missing check for software encoder

MediaCodecSource tries to allocate hardware codec for
video recording. If only software encoder is present
for a mime type, it results in a crash, while trying
to access an empty vector

Add change to allocate software encoder if hardware
encoder is not found

Change-Id: I629bfcf9d795559b9cdd46798580ee7900c9a25e
CRs-Fixed: 776726