fix broken Android build due to 617bd85

617bd85 broke the Android build by creating an unresolvable
order-of-operations bug in classpath-android.cpp's
MyClasspath::preBoot method.

The problem is that, while JNIEnv::FindClass is supposed to initialize
the class that it finds, this causes JniConstants::init to indirectly
invoke native methods which are not registered until JNI_OnLoad is
called (which happens after JniConstants::init is called).  However,
if we call JNI_OnLoad first, that causes methods to be invoked which
rely on JniConstants::init having already been run.

I haven't checked to see how Dalvik handles this, but I don't see any
way around the problem besides disabling initialization by
JNIEnv::FindClass until the preBoot phase is complete.  Moreover, it's
dangerous to allow Java code to be invoked so early anyway, since the
VM is not yet fully initialized.