mobile/bind: allow bound packages to refer to imported bound packages

Multiple packages are already supported, but only as if each packages
were bound in isolation. This CL lets a bound package refer to other
bound packages in its exported functions, types and fields.

In Java, the JNI class jclass and constructor jmethodID are exported
so other packages can construct proxies of other packages' interfaces.

In ObjC, the class @interface declarations are moved from the package
.m file to its .h file to allow other packages to constructs its
interface proxies.

Add a supporting test package, secondpkg, and add Java and ObjC tests
for the new cross package functionality. Also add simplepkg for
testing corner cases where the generated Go file must not include its
bound package.

While we're here, stop generating Go proxy types for struct types;
only Go interfaces can be implemented in the foreign language.

Change-Id: Icbfa739c893703867d38a9100ed0928fbd7a660d
Reviewed-on: https://go-review.googlesource.com/20575
Reviewed-by: David Crawshaw <crawshaw@golang.org>

bind/testdata/structs.java.h.golden

@@ -8,6 +8,13 @@
 
 #include <jni.h>
 
+extern jclass proxy_class_structs_I;
+extern jmethodID proxy_class_structs_I_cons;
+
 void cproxystructs_I_M(int32_t refnum);
 
+extern jclass proxy_class_structs_S;
+extern jmethodID proxy_class_structs_S_cons;
+extern jclass proxy_class_structs_S2;
+extern jmethodID proxy_class_structs_S2_cons;
 #endif