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.
fix SSL stack and Android JAR resource loading
fix Android thread enumeration
Small fix for Android Classpath on MacOS
Fixed jar tool usage - we should call "$(jar)" instead of simply "jar"
Fixed missing __BEGIN_DECLS / __END_DECLS on MinGW
Fixed CFLAGS for Android classpath under MinGW
Makefile fix for new ICU4C on Windows
a couple of tweaks to get Mac/Android tests passing
add Avian_java_lang_Thread_nativeSetName stub
fix segfault in BuiltinElement::dispose

If a BuiltinElement is never used (i.e. we never try to load anything
from it), its init method will never be called, and thus its backing
library will never be loaded.  In that case, we should not try to call
library->disposeAll() in the destructor.
Merge pull request #336 from dicej/builtinelement-dispose

fix segfault in BuiltinElement::dispose
update README.md to reflect conscrypt fork
implement JVM_isNaN

This method was previously unimplemented and untested.  However, the
JMH benchmark suite requires it, so now it's time to implement it.
Merge pull request #337 from dicej/master

implement JVM_isNaN
abort if JIT triggered when AVIAN_AOT_ONLY is defined

If AVIAN_AOT_ONLY is defined and the VM is asked to compile a method
anyway, we should abort immediately rather than let it crash later
(e.g. due to an access violation on a non-jailbroken iPhone).  This
makes debugging such issues a bit easier since the failure will come
earlier and with a more obvious cause.
Merge pull request #339 from dicej/aotonly

abort if JIT triggered when AVIAN_AOT_ONLY is defined
Merge remote-tracking branch 'origin/master' into android-upgrade
make classpath native sources depend on vm-depends in makefile (fixes #340)
Merge pull request #341 from joshuawarner32/master

Classpath native sources to depend on vm-depends in makefile (fixes #340)