Move implementations to DataSourceBase.h

Some lib*extractors include DataSourceBase.h but are not
linking libstagefright which has the implementation.
This doesn't cause any build issue in normal build, but
it actually matters when NATIVE_COVERAGE=true which uses -O0.

Fixing by moving the implementation to the header.

Bug: 139459652
Test: m
Test: NATIVE_COVERAGE=true COVERAGE_PATHS="*" m
Change-Id: Ic955189ff9f0fbc48b5b35eccdb2916dc79fdb0f