adb: run reverse_service() in main thread.

reverse_service() calls handle_forward_request(), which calls
functions in fdevent.cpp. fdevent functions is only supposed
to be called in the main thread.
Add check in fdevent.cpp to make sure all operations come from
main thread.

Bug: 25355808
Change-Id: Iceb9273f3056acc0713eaafe086ac950ca80ff4f