fix: Compile classes.dex as part of accesskit_android's build process

[DataTriny]

Inspired by this article, here's an attempt at generating our embedded dex file during compilation of the Android adapter.

• The build script does nothing if the embedded-dex Cargo feature is not enabled.
• Changes to Delegate.java are tracked by Cargo to only recompile when necessary.
• The generation process should be almost identical to the previous bash script, except it's portable now.
• The clippy CI job now has an additional step to check accesskit_android with the embedded-dex Cargo feature enabled, to make sure the build script at least compiles.

I've only tested this on my Windows 11 machine, Linux (Ubuntu) will be covered by the CI.