RootCanal: Fix potential use-after-free issues

The tasks registered within LinkLayerController capture
the pointer to the Controller instance for member access.
These tasks can outlive the Controller instance if the
controller is deleted (e.g. through the test channel, or
because the TCP connection is lost). In this case
attempting to execute the task callback would cause a
use-after-free invalid pointer access.

This change bypasses the issue by capturing a weak pointer
to the instance to check if it is live before running the
task callback.

Test: m root-canal && launch_cvd
Change-Id: I5384fc056fc4fbc0095b99ef7113fd4f706803ba