[TIMOB-1579] Memory leak: XML DOM NodeProxy never releases proxy objects from cache on Android

I've been hitting terrible out-of-memory errors while working on our StatusNet Mobile client app on Android. On some investigation, it seems that the XML-related code is leaking a lot of memory:

The caching code in NodeProxy.getProxy() says:

// We cache node proxies so we're not constructing new ones on every single call
// on node finalize we have to go back through and remove each proxy

but I don't see any finalizer on NodeProxy and friends.

Dumping the proxy objects into an ever-growing HashMap means that DOM-related objects never get garbage collected once they've been accessed from JS code: both the proxy and the native object are always referenced from the HashMap.

Simply removing the addition of proxy objects into that proxyCache hash map makes my app's memory usage MUCH more stable. If the cache is necessary, weaker references should probably be used to ensure that the garbage collector can release the proxy objects.