Avoid porting null keys when copying preferences into new system

Testing on Fishlake shows certain preferences are of 'null' type
in shared preferences. Avoid an NPE on porting a null. From testing,
this can happen with focus mode, best to just skip porting the key
and let the app use a default.

Bug: 17829562
Change-Id: I483cb5beb160d3fb7fc8a42aa5085531e3fccb63
1 file changed