Always create Debug thread (but start on 1st use)

AdbDebuggingHandler tries to start/stop the debugging thread on demand.
This causes several problems:
 - There is a race condition where the thread can be started and a
   message is sent right away before the connection has been
   established. In this case the message is lost.
 - There are several edge cases where the start/stop counter is not set
   properly which results in the thread being stopped but messages are
   sent anyway.

This CL always creates the thread object but only starts it once when
adbd is enabled.

Test: NA
Bug: NA
Flag: EXEMPT(refactor)

Change-Id: I324bf37bb34a768a013df4bc31468adf3922b561