Fix Settings crash when setting only decimal point to data usage warning

Settings app crashes when only decimal point (".") is set to
data usage warning because it is not a valid number.
To fix this issue, the decimal point should be converted to 0.

Fixes: 63787824
Test: manual - go to Settings > Data usage > Mobile data usage and then
               set only decimal point to data usage warning.

Change-Id: Iefb6a035ba6f17b8e0c89f79dbd397e2356fb19c