Add framework-mime-sources to stubs generation

It was used in the docs stubs generation but it doesn't have its own
(jar) stub generation. Add it to the shared non-updatable sources for
consistency.

The only class in these sources is @hide, so it makes no
immediate difference.

Test: diff android_stubs_current artifact before/after
Change-Id: I33e75623d8d71da362fb16befb78c8c7596fa400