[Maintenance] Update NDK dependencies

[kanerogers]

Background

There's been some activity (pun intended) in the ndk crates. rust-mobile/ndk#372

In short, it looks like:

• ndk-glue has been deprecated in favour of https://github.com/rust-mobile/android-activity
• ndk now supports newer versions of the NDK (important, as we're on a pretty old version)

In addition:

• oboe now uses ndk 0.7 (which we were previously blocked on)

This is a good thing, as it should hopefully prevent the landmines of having the NDK crates fall out of sync.

So, what we'll need to do is:

• Bump our cpal dependency to get the newer version of oboe
• Switch to android-activity

[kanerogers]

Switching to android-activity seems like a good idea, it'll just take a bit of work. Here's the API for android_activity

use android_activity::AndroidApp;

#[no_mangle]
fn android_main(app: AndroidApp) {
  loop { app.poll_events(..) }
}

This design is a lot cleaner and more - dare I say - it Rusty than ndk-glue as it doesn't require any magic global variables.

However, this would require some updates to Engine as we'd need to pass AndroidApp into it and update its polling logic. Not impossible, but a bit tricky as we'd have to find a way to keep things cross-platform