Prevent MediaScanner from removing Micro thumbnail database after scan.

After scanning the sdcard for the first time, the MediaScanner deletes
the micro thumbnail database files. The MiniThumbFile class attempts to
recreate the files but ends up with 0-sized db-files that it cannot
store any more micro thumbs in, resulting in that all micro thumbnails
that are results from thumbnail request are never stored properly until
the device is restarted.

Change-Id: I2b49580ae3360447525dbee80f6ba15b4bc658dd