Mark libmedia_datasource_headers available to platform

The lib is indirectly depended on by mediaextractor and some more that
are part of the platform. One of the dependency chains is extractractor
-> libmediaextractorservice -> libstagefright -> ligstagefright_id3 ->
libmedia_datasource_headers.

With b/153073816, we will be checking the availability to the platform,
i.e. platform-available module depending on platform-unavailable module
causes a build error.

Prepare for the change by correctly marking that this interface lib is
available to the platform.

Bug: 153073816
Test: m
Change-Id: Id7bc146cb4712fc1b2d5c00666ad3750b9663295