Add fast path for FindClass using the type dex file.

If we are using the PathClassLoader with a BootClassLoader
parent, we can handle the common case in the FindClass function
without needing to go back to java code.

Around 10% speedup measured of maps launch, could be noise due to
large variation of app launch times. Eliminates defineClassNative
from being anywhere near the top of sampling profiles.

Bug: 17397179
Bug: 16828525
Change-Id: Ide0db2b5f6cf5b96fc46e89178e0799de667cb88