add android NDK event loop

[JasperDeSutter]

• Tested on all platforms changed
• Compilation warnings were addressed
• cargo fmt has been run on this branch
• cargo doc builds successfully
• Added an entry to CHANGELOG.md if knowledge of this change could be valuable to users
• Updated documentation to reflect any user-facing changes, including notes of platform-specific behavior
• Created or updated an example program if it would help users understand this functionality
• Updated feature matrix, if new features were added or implemented

These are not my changes, this backports the work (goddessfreya#2) of @dvc94ch to winit master. I have no issue if they want to take over this PR.

Running an example on an android device needs a couple things, add this to Cargo.toml:

[[example]]
name = "request_redraw_threaded"
crate-type = ["cdylib"]

And add this to the example file:

#[cfg(target_os = "android")]
ndk_glue::ndk_glue!(main);

Now start the application with cargo apk run --example request_redraw_threaded

Closes #721, closes #789, closes #948 cc @katyo

[dvc94ch]

Great work thanks!

[dvc94ch]

@JasperDeSutter can you add the ci changes to this PR too?

[dvc94ch]

LGTM

[dvc94ch]

@JasperDeSutter it's on my todo list to merge before end of the week, let's just give it a couple more days incase someone wants to comment.

[JasperDeSutter]

Okay, sounds great!

[dvc94ch]

Needs rebasing.

I'm wondering if it makes sense to add the ndk glue to all examples.

#[cfg(target_os = "android")]
ndk_glue::ndk_glue!(main);

The other issue is that android apps rely on Suspend and Resume events being emitted. So a winit application that wasn't written with android in mind won't work. But if all backends emit Suspend and Resume events a winit application written for android can work on all platforms.