Fix crash when showing the device not found dialog.

If users click to pair a not foundable device and then close the Settings page,
Settings will crash becasue dialog holder is closed.
This CL is part of the fix. It set the context to null when the activity
stops.

Bug: 306548036
Test: atest
Change-Id: If68e3e9cd62216ad83b2e5058f504fce35e36a06