Vendor dependencies

Let's see how I like this workflow.

vendor/android_system_properties/README.md

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+# android_system_properties
+
+A thin rust wrapper for Android system properties.
+
+This crate is similar to the `android-properties` crate with the exception that
+the necessary Android libc symbols are loaded dynamically instead of linked
+statically. In practice this means that the same binary will work with old and
+new versions of Android, even though the API for reading system properties changed
+around Android L.
+
+## Example
+
+```rust
+use android_system_properties::AndroidSystemProperties;
+
+let properties = AndroidSystemProperties::new();
+
+if let Some(value) = properties.get("persist.sys.timezone") {
+   println!("{}", value);
+}
+```
+
+## Listing and setting properties
+
+For the sake of simplicity this crate currently only contains what's needed by wgpu.
+The implementations for listing and setting properties can be added back if anyone needs
+them (let me know by filing an issue).
+
+## License
+
+Licensed under either of
+
+ * Apache License, Version 2.0 ([LICENSE-APACHE](LICENSE-APACHE) or http://www.apache.org/licenses/LICENSE-2.0)
+ * MIT license ([LICENSE-MIT](LICENSE-MIT) or http://opensource.org/licenses/MIT)
+
+at your option.