Upgrade the Android build scripts and workflow

[thunderbiscuit]

This is a semi-substantial upgrade of the Android build system. Here's what's in this PR:

1. Upgrade of the AGP to 8.13.2 (9.0.0 has some bugs/conflicts with the Dokka plugin at the moment).
2. Upgrade of the Gradle wrapper to 9.3.1 (latest).
3. Clean up of our use of the library version and group variables, which were where they currently are because of historical reasons (our old publishing plugin which doesn't apply anymore because it doesn't work with the new Central Portal).
4. Add a new releasing plugin which makes my life much easier when releasing. I have used it for Kotlin-only projects and we have it merged on bdk-jvm.
5. Bump the Dokka docs to 2.1.0, which comes with some small but nice UI improvements.

Notes to Reviewers

You can test the new build and publishing workflows by running

cd bdk-android
./gradlew publishToMavenLocal

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran cargo fmt and cargo clippy before committing
• I've added a changelog in the next release tracking issue (see example)
• I've linked the relevant upstream docs or specs above

New Features:

• I've added tests for the new feature
• I've added docs for the new feature

Bugfixes:

• This pull request breaks the existing API
• I've added tests to reproduce the issue which are now passing
• I'm linking the issue being fixed by this PR

[ItoroD]

ACK 39feaf0

Published locally and checked out docs Looks good.