Added libmedia_datasource_headers header library

This exports the 'include' directory of libmedia only. It does
not export any additional header libraries. This can be used
by modules which require only header files from libmedia.

Test: Build libmedia_datasource_headers
Bug: 151789258

Change-Id: I1814f07e29ea0b8a7db6e7f8657ffcba6ef582e7