Added "libmedia_ndkformatpriv" static library

Note: This contains the implementations required by
"libmediandk_format" library. This is built standalone
to avoid copy into ndk folder.
This can be used by extractor's along with "libmediandk_format"
instead of 'libmediandk'

Test: Build libmedia_ndkformatpriv
Bug: 151789258

Change-Id: I60cd5bb626b222dfa8ca7bd66815b01d1131e0fd