Settings : Add input checks for blacklist input

There can be situations where user enters invalid input.
Ensure that is handled.

Repro :

1. Add new blacklist entry
2. Enter /
3. Press ok
4. Enjoy crash

issue-id: CYNGNOS-980

Change-Id: Ica90ac835f374569123e679ba39c3caa7179ef65
(cherry picked from commit 23612493)