libstagefright: Create MP3 decoder libraries without OMX layer

- With the current MP3 OMX SW decoders, the decoding time
is increased w.r.t the libraries without OMX layer that are
present in GB. This increase in decoding time results reduction in
power savings in LPA mode.
- This commit is to remove OMX layer for MP3 to reduce the
power consumption in LPA mode

Change-Id: Ie194b11619de9ba43cd85dddbf7892239858945d