Split datasource out of libstagefright

into libdatasource, which contains:
  DataSourceFactory
  (Clear)FileSource
  (Clear)MediaHTTP
  DataURISource
  HTTPBase
  NuCachedSource2

This is needed to break a circular dependency in an upcoming CL.

Test: build, boot

Change-Id: I34d9937235c78f18f51b18945342a0743e209577