Avoid memory leaks when handling metadata strings

Avoid memory leaks when handling metadata strings

Don't duplicate strings when retrieveing metadata from media
files. As any requests for metadata strings would pass through
the binder, this would cause the reference to the duplicate string
to be lost, causing a memory leak as the duplicate would not be
freed.

Change-Id: I2593733472b1bb589bc502b2c11080f581015bb5