A first step towards replacing the android_native_app_glue code with a
pure Rust implementation.

A literal port for now, with the intention of re-working into more
idiomatic Rust code once we have a port of all the C code.

At this point the C code has been fully ported to Rust but it's not yet
well integrated with the pre-existing Rust code and the port is not yet
particularly idiomatic Rust code.

This is a fairly thorough re-working of all the code that was initially
naively ported from android_native_app_glue.c to be more idiomatic Rust
code (though by it's nature it still involves a lot of unsafe code)

Most of the glue code now lives in src/native_activity/glue.rs as part
of a NativeActivityGlue API

The design as far as the threading model, IPC and synchronization goes
is unchanged.

…ogging)

…for callbacks

This adds a common `try_with_waitable_activity_ref` utility that handles
upgrading the Weak reference associated with `ANativeActivity` whenever
we get an activity callback.

Previously we weren't converting the Weak reference back to a pointer
before returning from the callback which would result in us dropping the
Weak reference after the first callback.

This simplifies the tracking of saved state by simply using a Vec and
only copying into a `malloc()` allocation when passing the saved state
to `ANativeActivity`.

This also ensures saved state persists (in Rust) between a
`MainEvent::SaveState` event and a `Resume` event - otherwise the saved
state would only be restored in the situation where `onCreate` is called
again for a new process.