Support scanDirectories with JNI callback function

MtpServer needs this function to update the database for directory
copy operation.

Test: Call this function from MtpServer and database is updated in
directory copy scenario.

Change-Id: I9b316f4dbf7f35922292c33ab02f9efebc867aa8
Signed-off-by: kyle_tso <kyle_tso@htc.com>