Fix a race condition in ZenModeTest

We should wait for the real change of the ZEN_MODE/AIRPLANE_MODE_ON
settings instead of wait for the broadcast. Otherwise there can
be race condition, i.e., after the broadcast happened we check if
the value is changed to expected value, but the change may not happen
yet.

Bug: 143038550
Test: atest CtsVoiceSettingsTestCases
Change-Id: I7873f94e985a9e4e16e73081e77b8178e977f331
Merged-In: I69f31275ecb44fe05f9c1cd0c8d5b3eab5a3fd4b
(cherry picked from commit 9c5539572191dcd546ee41f12cac901c6da543e0)