Android example: document pre-requisites and fix code for rust 1.92.0 (#8122)

* [x] I have followed the instructions in the PR template

```
+ echo 'All checks passed.'
All checks passed.
```

I had to update `.png`'s for tests to pass, but I guess it's specific to
my laptop.

---------

Co-authored-by: Emil Ernerfeldt <emil.ernerfeldt@gmail.com>

examples/hello_android/README.md

@@ -1,24 +1,31 @@
-Hello world example for Android.
+# Hello world example for Android.
 
-Use `cargo-apk` to build and run. Requires a patch to workaround [an upstream bug](https://github.com/rust-mobile/cargo-subcommand/issues/29).
+## Desktop pre-requisites
 
-One-time setup:
+Android application require you to have toolchains and Android SDK installed:
 
-```sh
-cargo install \
-    --git https://github.com/parasyte/cargo-apk.git \
-    --rev 282639508eeed7d73f2e1eaeea042da2716436d5 \
-    cargo-apk
-```
+1. `rustup target add armv7-linux-androideabi aarch64-linux-android` - install targets for android
+2. Set environment variables (these variables are required each time for `cargo apk`):
+  ```sh
+  export ANDROID_HOME="$HOME/tools/android"
+  export ANDROID_NDK_ROOT="${ANDROID_HOME}/ndk/29.0.14206865"
+  export PATH="$PATH:${ANDROID_NDK_ROOT}:${ANDROID_HOME}/build-tools/${BUILDTOOLS_VERSION}:${ANDROID_HOME}/cmdline-tools/bin"
+  ```
+3. Install command line tools:
+  ```sh
+  mkdir -p "${ANDROID_HOME}/cmdline-tools"
+  curl -sLo /tmp/clt.zip https://dl.google.com/android/repository/commandlinetools-linux-14742923_latest.zip
+  unzip -d "${ANDROID_HOME}" /tmp/clt.zip
+  ```
+4. Install SDK components: `sdkmanager --sdk_root="${ANDROID_HOME}" --install "build-tools;36.0.0" "ndk;29.0.14206865" "platforms;android-35"`
+  > You may need to change SDK versions
+5. Install cargo-apk: `cargo install --git https://github.com/parasyte/cargo-apk.git --rev 282639508eeed7d73f2e1eaeea042da2716436d5 cargo-apk`
+  > There was a [bug](https://github.com/rust-mobile/cargo-subcommand/issues/29) in the upstream, so the above installs a patched version. 
 
-Build and run:
+## Build & run
 
-```sh
-# Run on android
-cargo apk run -p hello_android --lib
-
-# Run on your desktop
-cargo run -p hello_android
-```
+Use `cargo-apk` to build and run:
+1. On android: `cargo apk run -p hello_android --lib`
+2. On desktop: `cargo run -p hello_android`
 
 ![](screenshot.png)

examples/hello_android/src/lib.rs

@@ -3,7 +3,7 @@
 use eframe::{CreationContext, egui};
 
 #[cfg(target_os = "android")]
-#[no_mangle]
+#[unsafe(no_mangle)]
 fn android_main(app: winit::platform::android::activity::AndroidApp) {
     // Log to android output
     android_logger::init_once(