Settings: fix the NullPointerException when initiating media player

If the input media resource is invalid, MediaPlayer create calling may
fail and returns null, so trigger one NullPointerException.

Add the null pointer judegement when calling seekto and so on.

Test: manual
Fixes: 130032215

Change-Id: I38cb08584aa4e4c2ce4dc58a5a095960c7bd1948
Signed-off-by: Hoff Cheng <hoffc@qti.qualcomm.com>