Fix `--secondary-class-loader-context` for run-tests.

Change the default context to flat PathClassLoader if the
`--secondary` flag is passed and keep the old PCL hierarchy
otherwise. Explicitly pass that flag in tests where this
default is not correct.

Test: testrunner.py --host --optimizing
Test: Make the "ClassLoaderContext classpath size mismatch"
      warning FATAL and retest; only 172-app-image-twice
      fails and the failure is expected for that test.
Bug: 166088871
Change-Id: Ic50f206741e6d1f98e08b5db2adc2d7b067266a7
7 files changed