AudioPolicyDeathTest: fix flakiness

In some contexts, launching the activity that registers
the dynamic audio policy can take more than 1 second which
causes the test to fail.
Add a callback confirming that the activity is running and
wait for the callback before proceeding with the test.

Bug: 206614157
Test: atest AudioPolicyDeathTest
Change-Id: Iac892e7a679c99973af5a1fe15b7b189299c4b47