adb: move ownership of the fdevent thread into FdeventTest.

Previously, each of the tests was spawning the fdevent thread manually,
in order to be able to set up listeners and such before running
fdevent_loop. Now that we have a way to run arbitrary code on the
fdevent thread, switch to having a generic fdevent thread and running
setup code via fdevent_run_on_main_thread.

Test: adb_test
Test: wine adb_test.exe
Change-Id: I517dbcbad31067b45087d9fbed67a75b75a75aec