libstagefright: use appropriate hevc decoder on 8916 and 8939

8916 supports HEVC software decoder and 8939 support h/w decoder.
Based on media.swhevccodectype and media.hwhevccodectype properties values
instantiate appropriate HEVC decoder.

Change-Id: Ic3d37e6184db256f4d83bfe74b67271b20f94ec0