Use2GOnlyCheckBoxPreference: fix NPE

Change-Id: Iacedd9f8eefb2d7f8ecdec4c0f1595448edbaf9f
Show checkbox for message wait indicator for everyone

This will show the settings checkbox for all devices regardles of
the sprint_mwi_quirk overlay settings.

The current logic was to hide the checkbox if the overlay was
missing or set to false. Additionally the default value was false,
which will never show an indicator.

Change-Id: I627071f4d83d3416c905161f1876cffd97dd50f1
Signed-off-by: Dirk Rettschlag <dirk.rettschlag@gmail.com>