Avoid invalid memory access when using extractMetadata()

BpMediaMetadataRetriever::extractMetadata() returns a string
which is returned by readCString() function of the reply parcel object.
However, the parcel object is destroyed at the end of the
extractMetadata() function, and so the returned pointer is invalid.
This patch fixes this problem by storing the metadata string value.

Change-Id: I83fdd9c81f8cae7fcc9fd3315eb1fb943d68a5d9
Signed-off-by: Sangkyu Lee <sk82.lee@lge.com>
Merge "Avoid invalid memory access when using extractMetadata()" into jb4.2