bluetooth: Don't crash when the local adapter is not set

In devices where bluetooth is not working (like the emulator or devices
in development), the bluetooth Settings crashes when closing the activity.

Most of the code checks for the nullability of the local adapter except
for this particular piece. This is usually not a worry for production
devices and I wouldn't usually worry about this but since the rest of the
code performs the nullability check it seems silly to crash onPause.

Change-Id: I7c1d3b43969b516336be770957008ef2054d312f
TrustedCredentials: Don't crash when the activity is paused

If you press Settings -> Security -> Trusted Credentials, then press
 back before the loading finishes, the loader async task will still
try to dereference the activity which will cause an NPE.

This patch fixes that by getting and holding a reference to the
context in the beggining and bailing early if it is null.

Change-Id: Ie03734d5f184821df5d8729fc4a16eb51cdeebf5
bluetooth: Show a refresh icon on the select devices screen

The refresh menu is hidden behind an overflow which is
pointless. This patch shows the refresh icon if there is
enough room.

Change-Id: I35f19c42054b05388f7b8fcb82e0f3bd60e2d618