Only init ndk-context once with an Application ref

[rib]

Instead of initializing ndk-context with an Activity reference (for the android.context.Context subclass) we now initialize with an android.app.Application reference (also an android.context.Context subclass).

The benefit of this is that we can strictly initialize ndk-context once (via a OnceLock) so there's no risk of a panic in case an application starts more than one Activity within the same process.

For reference; I've tested this change in conjunction with amodm/webbrowser-rs#111 to confirm that the webbrowser crate can be made to work with an Application or Activity reference (the currently implementation depends on having an Activity reference, even though ndk-context doesn't promise to provide an Activity as the Context).

Fixes: #58
Fixes: #228