CI: add _android-ci-activity feature to select NativeActivity class

Normally an application crate based on android-activity should specify
the Activity class they are using via the "native-activity" or
"game-activity" features.

The choices are mutually exclusive with no default and the
android-activity crate will emit a `compiler_error()` if no class has
been chosen.

It's important that Winit (and other middleware crates that may link
against android-activity) do not impose a default class choice,
considering that cargo features are additive.

For CI to be able to compile Winit standalone this adds a private
`_android-ci-activity` feature that will select the NativeActivity
class as if building an application crate.

.github/workflows/ci.yml

@@ -33,7 +33,7 @@ jobs:
           - { target: x86_64-unknown-linux-gnu, os: ubuntu-latest,   }
           - { target: x86_64-unknown-linux-gnu, os: ubuntu-latest, options: --no-default-features, features: x11 }
           - { target: x86_64-unknown-linux-gnu, os: ubuntu-latest, options: --no-default-features, features: "wayland,wayland-dlopen" }
-          - { target: aarch64-linux-android,    os: ubuntu-latest,  cmd: 'apk --' }
+          - { target: aarch64-linux-android,    os: ubuntu-latest,  cmd: 'apk --', features: "_android-ci-activity" }
           - { target: x86_64-apple-darwin,      os: macos-latest,    }
           - { target: x86_64-apple-ios,         os: macos-latest,    }
           - { target: aarch64-apple-ios,        os: macos-latest,    }

Cargo.toml

@@ -42,6 +42,11 @@ wayland-csd-adwaita = ["sctk-adwaita", "sctk-adwaita/ab_glyph"]
 wayland-csd-adwaita-crossfont = ["sctk-adwaita", "sctk-adwaita/crossfont"]
 wayland-csd-adwaita-notitle = ["sctk-adwaita"]
 
+# Normally the app specifies the Android Activity class, but we have this feature
+# for CI, to select NativeActivity (otherwise android-activity will emit
+# a compiler_error() if no class has been chosen).
+_android-ci-activity = [ "android-activity/native-activity" ]
+
 [dependencies]
 instant = { version = "0.1", features = ["wasm-bindgen"] }
 once_cell = "1.12"