Fix b/5353417 Images downloaded from browser are showing incorrect details...

Fix b/5353417 Images downloaded from browser are showing incorrect details (Time is shown as dec 31 1969)

MediaScanner sets DATE_TAKEN with EXIF's datetime tag value. When this information is not available,
ExifInterface will simply return -1 which is accidentally used by MediaScanner.

Adding a check to avoid it so MediaProvider can calculate the date taken from last modified time instead.

Change-Id: I305b93a6c5602cbb9f97c3bbd384d358bda030c6