Audio VTS: Improve stability, update config validation test

Major challenge: the configuration of HAL tests
depends on the APM XML configuration file which may be
invalid. The code that reads the configuration
has been updated to avoid crashes when the config
is invalid.

In CheckConfig_audioPolicyConfigurationValidation the
dependency on the config parser was removed. Previously,
a failure to parse the config by the config parser would
lead to the test being skipped, which isn't correct
as it must fail in this case.

Minor fixes to V7 tests to pass on a real life legacy HAL.

Bug: 36733185
Test: run VtsHalAudioV7_0TargetTest on a device with V6
Test: run VtsHalAudioV7_0TargetTest on a device with
      a side-loaded V7 and invalid APM config file
Test: run VtsHalAudioV7_0TargetTest on a device with
      a side-loaded V7
Change-Id: I746339ff69ab455dc64eef9a17827d047b357329