Audio VTS: Always check for Result::isOk()

A test was not checking for the Result::isOk() resulting
in an assert being triggered in HIDL code if the HAL crashed.

Test: (sleep 0.3; adb shell pkill audio) &
      adb shell /data/nativetest64/VtsHalAudioV4_0TargetTest/VtsHalAudioV4_0TargetTest
      # Test finishes in failure but does not crash
Signed-off-by: Kevin Rocard <krocard@google.com>

Change-Id: I97a6213fe82a5e9a6eb333dc1dc2476102610b58