adbd: avoid starting multiple worker threads.

A previous patch intended to make receiving multiple FUNCTIONFS_ENABLEs
non-fatal, but failed to do so because we would try to spawn another
worker thread and hit a different assertion.

Bug: http://b/130638368
Test: echo mem | adb shell "su 0 sh -c 'cat > /sys/power/state'"
Change-Id: I53456112244d8b4f7d26df6ec6961389fca70498