Fix bugs in ravenhelper-PTA

- Load nested-nested-classes properly
- Fix potential NPE when a class isn't found

Bug: 292141694
Flag: TEST_ONLY
Test: $ANDROID_BUILD_TOP/frameworks/base/ravenwood/scripts/run-ravenwood-tests.sh -r
Change-Id: Ic95b436c566cc77324463a3dfb4a411f4ae8a477