Spawn background verification for InMemoryDexClassLoader

After dex files have been loaded with InMemoryDexClassLoader, call
DexFile.verifyInBackground which instructs ART to spawn a background
thread and verify all newly loaded classes. This thread will be used
for creating a vdex file to speed up subsequent loads of the same
bytecode.

Bug: 72131483
Test: art/tools/run-libcore-tests.sh
Test: art/test.py -b -r -t 692
Change-Id: I1de0cca3e7e4739cd5f1929d66c53a538067b8e5