Separate libstagefright_omx into two libs

Extract the part in libstagefright_omx that's the base
of all soft omx components into a separate lib,
libstagefright_omx_soft, and make the omx plugin libs
only depend on this lib and can be built more independently.

This way the bufferqueue/gui reference in omx server side
will be restricted to libstagefright_omx, which hosts
only OMX master and node instances.

Also remove unnecessary gui references in some soft plugins,
they don't have a real dependency on gui.

bug: 129272021

Change-Id: I81f789e44c4133579230bff0e0fd8633d349ecc4