update to more recent version of Android class library

Lots has changed since we forked Android's libcore, so merging the
latest upstream code has required extensive changes to the
Avian/Android port.

One big change is that we now use Avian's versions of
java.lang.Object, java.lang.Class, java.lang.ClassLoader, some
java.lang.reflect.* classes, etc. instead of the Android versions.
The main reason is that the Android versions have become very
Dex/Dalvik-specific, and since Avian is based on Java class files, not
dex archives, that code doesn't make sense here.  This has the side
benefit that we can share more native code with classpath-avian.cpp
and reduce the amount of Java/C++ code duplication.