[Companion] Fix NPE in associate

The root cause of the exception was that the activity destroy listener was
reacting to any activity being destroyed instead of just the one used with
the CompanionDeviceManager

Fixes: 62549525
Test: Ensure the attached bug no longer reproduces
Change-Id: I2f977e9ac9176247f5be9d08d19b3875f2b4a703
1 file changed