Revert "Wait for an abort event when stopping a model"

This reverts commit 6e2eb81a.

The original commit fixed b/223922855, which resulted from fixing
b/191935600, but created another.
Blocking the stop() call until the event confirming the stop arrives is
a bad idea, since:
- We have to give up the lock when waiting for the event, and thus lose
  the atomicity of some of the operations. b/226926627 is an example of
  this.
- It turns out some of the forward calls happen from the main thread's
  looper, which is also used for callbacks, and we risk deadlocks.

A follow-up commit will address the fix in a better way.

Bug: 191935600
Bug: 226926627
Bug: 223922855
Test: See next commit in the chain.
Change-Id: I2b9987ae93803d3b4f72a1df56c88e284bed43c5