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.
Merge pull request #229 from dicej/preBoot

fix broken Android build due to 617bd85