Keyguard: do not allow to delete the system widget if widgets are disabled

We overwrite the default widget with our own one. In this case it is detected
as a normal widget even though it widgets are disabled and starts listening to
longpress events

Whatever check if widgets are disabled. If this is the case do not allow any
longpress events

Change-Id: I471664bac6ca1f8e57efe4904a385904c40486db
Fix preference puts with "null" values.

Null values were being written out as <null /> elements in the
XML prefs file (as expected). This allowed the getFoo() functions
to work correctly because they treated null values as missing mappings
but containsKey would fail.

bug: https://code.google.com/p/android/issues/detail?id=64563
Change-Id: I1f466d01db96bf26e208d4fed3a6f257228bea5d