fix all the bugs

So there I was, planning to just fix one little bug: Thread.holdsLock
and Thread.yield were missing for the Android class library.  Easy
enough, right?  So, I added a test, got it passing, and figured I'd go
ahead and run ci.sh with all three class libraries.  Big mistake.

Here's the stuff I found:

 * minor inconsistency in README.md about OpenSSL version

 * untested, broken Class.getEnclosingMethod (reported by Josh)

 * JNI test failed for tails=true Android build

 * Runtime.nativeExit missing for Android build

 * obsolete assertion in CallEvent broke tails=true Android build

 * obsolete superclass field offset padding broke bootimage=true Android build

 * runtime annotation parsing broke bootimage=true Android build
   (because we couldn't modify Addendum.annotationTable for classes in
   the heap image)

 * ci.sh tried building with both android=... and openjdk=..., which
   the makefile rightfully balked at

Sorry this is all in a single commit; I didn't expect so many
unrelated issues, and I'm too lazy to break them apart.

classpath/avian/Pair.java

@@ -0,0 +1,5 @@
+package avian;
+
+abstract class Pair {
+  // VM-visible fields in types.def
+}