DRAFT! Parse Intent from getIntent() and provide extra getters

[MarijnS95]

Depends on #9

Works towards rust-mobile/ndk#275 / rust-mobile/ndk#380, but doesn't yet fully implement the onNewIntent() callback + setIntent() yet to follow up on new intents delivered to an already running app.

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Quick draft to see if I could use this crate. While there's exactly zero overlap in the current implementation (this crate only provided setters, whereas I only needed getters), I still think this is the right place to implement (continue implementing...) a JNI-based Intent wrapper. No other Rust crate exists for this purpose yet while it's still very useful to have in Rust.

This renames Intent to IntentBuilder (because of storing a "builder-pattern" Result inside) and Inner to a publicized Intent where a few new getters are implemented. A free with_current_intent() function is added to provide a view of an 'env scoped Intent based on Activity.getIntent() on ndk_context::android_context() (I know, broken design).

TODO

• Replace .unwrap() with proper result wrappers.
• Include more getters.
• Move setters to the Intent struct where they return normal Results, which can then be captured in the IntentBuilder instead.
• Clean up commented mess :)