Refactor the android-toolchain project to include only the common items

We download a lot of stuff in android-toolchain.

However, I think that part of it is used only by GeckoView.

We should see if we can move it to GeckoView, or create some projects for stuff that is used by various projects (or during singing, such as the build tools).

The reason is that some updates (not to mention errors) on the android-toolchain project might trigger unneeded complete rebuilds of projects such as Rust and Clang.

/cc @boklm @brizental