gd: Fix reactor race condition

If one thread is in Unregister() while another is
in Run(), there is a very small window where
the reactor lock drops allowing Unregister()
to think the reactable is not running, but it is.  So
then the reactable is deleted underneath the Run()
thread and crashy ensues.

Bug: 148703760
Test: bluetooth_test_gd --gtest_filter=L2capClassicLinkTest.* --gtest_repeat=1000000
Change-Id: I11535fdcda2102102e6e9e23baecd5465549c723