mediaextractor: ensure no direct open()s

Mediaextractor should only be operating on data passed directly to it.
It shouldn't be attempting to open /data files on it's own.

Add a neverallow statement (compile time assertion + CTS test) to ensure
this is the case.

Bug: 67454004
Test: policy compiles. No runtime impact.
Change-Id: Ie94d4cb9aece7e72fbd13321f339dcf9d44d5d77