[feedback] use a share_ptr for the LogListener fit::bridge to share it with the async loop

* we use a fit::bridge in our LogListener to complete the log collection upon Done(), connection loss or timeout
* the timeout is implemented via a delayed task we post on the async loop that completes the fit::bridge
* we need to share that fit::bridge, otherwise the LogListener could go out of scope (upon Done()) and the async loop is still trying to complete the fit::bridge 10s later

FLX-179 #done #comment

TESTED=`fx run-test feedback_agent_tests -t feedback_agent_integration_test -- --gtest_filter=FeedbackAgentIntegrationTest.GetData_CheckKeys --gtest_repeat=10000` (the issue appears 10s after the first run so usually 1000 times does the trick or manually decrease the 10s timeout)

Change-Id: Idd6bd62938dd9abd3b67a058076a3de6eb1a4f28