Persistent settings (SettingsPlugin, SettingsGroup, ...) does not work on Android

## Bevy version and features

- Bevy 0.19.1

## What I did

I used bevy persistent settings, in a very similar way to the `app/settings.rs` example, it works on Linux, but not on Android.

## Additional information

I worked around the issue by modifying `bevy_platform/src/dirs/mod.rs` to become
```rust
//! APIs that return the location of standard user directories.

// Modeled after https://github.com/dirs-dev/dirs-sys-rs/

#[cfg(not(any(target_os = "windows", target_os = "macos", target_os = "linux")))]
use std::path::PathBuf;

#[cfg(target_os = "windows")]
mod windows;
#[cfg(target_os = "windows")]
pub use windows::preferences_dir;

#[cfg(target_os = "macos")]
mod macos;
#[cfg(target_os = "macos")]
pub use macos::preferences_dir;

#[cfg(target_os = "linux")]
mod linux;
#[cfg(target_os = "linux")]
pub use linux::preferences_dir;

#[cfg(target_os = "android")]
pub fn preferences_dir() -> Option {
Some(PathBuf::from("/data/data"))
}

/// Returns the path to the directory used for application settings. This version
/// always returns `None`.
#[cfg(not(any(
target_os = "windows",
target_os = "macos",
target_os = "linux",
target_os = "android"
)))]
pub fn preferences_dir() -> Option {
None
}
```
The important added part being
```rust
#[cfg(target_os = "android")]
pub fn preferences_dir() -> Option {
Some(PathBuf::from("/data/data"))
}
```
I have almost no Android experience and I am not 100% sure this is the best way to do this on Android but "it worked for me". I will make a PR as soon as possible. Like kfc35 suggested on Discord: I will put the android specifics in a separate file (even if almost empty like it is for macos) and not in the `mod.rs`

Start with bevy_platform/src/dirs/mod.rs and the app/settings.rs example, then review how platform-specific directory implementations are organized, including the separate macOS file. Add Android-specific handling without hard-coding an unsuitable path, and verify that persistent settings work in the Android version of the example.

Written by the indexing model from the issue text.