Don't export libogg symbols from libstagefright

Some of these are also exported from libvorbisidec (Tremolo), which
can lead to strange results and crashes when linking with both
libstagefright and libvorbisidec.
Not exporting these symbols also shaves a few kilobytes off the size
of libstagefright.so

Test: build, run

Change-Id: I1ee6deb4fff11ff66063648d7681a7e25f4a2b86