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

(cherry picked from commit 8a39e7fe)

Change-Id: I1f31de76c1b1a53e32173b1e61a59b0e2f267c98