audio: make sure to set back to AudioMode::NORMAL after the test case

If the major version is greater than or equal to 6, the test tries
to set the mode to AudioMode::CALL_SCREEN. However, it doesn't set
back to AudioMode::NORMAL. Replace all ASSERT with EXPECT to ensure the
test can reach to the explicit call to reset to AudioMode::NORMAL.

Bug: 194022995
Test: atest VtsHalAudioV7_0TargetTest
Change-Id: Ib9b6e310965a85b016853b72c60716fa054641c6