JavaKit: always use ambient javaEnvironment on Android

This is not a real fix, I suspect the real fix is to remove javaEnvironment as
an instance variable on all platforms and always use the ambient environment
(as the JNI specification clearly states the environment cannot be shared
between threads).

Works around: swiftlang#157

Sources/JavaKit/JavaObjectHolder.swift

@@ -25,7 +25,11 @@ public class JavaObjectHolder {
   /// so that the Java virtual machine will not garbage-collect it.
   public init(object: jobject, environment: JNIEnvironment) {
     self.object = environment.interface.NewGlobalRef(environment, object)
+#if canImport(Android)
+    self.environment = try! JavaVirtualMachine.shared().environment()
+#else
     self.environment = environment
+#endif
   }
 
   /// Forget this Java object, meaning that it is no longer used from anywhere