Don't compile in dependencies that aren't used by given tool

[dupontgu]

Noticed this while trimming my own tool and it seems worth flagging, since it
affects everything built on the SDK.

:sdk:ui includes LightQrCodeScanner, which pulls in ML Kit's barcode model.
libbarhopper_v3.so is 19.3MB of a 29MB release APK — about two thirds of the
download — and it ships for four ABIs on a phone that's arm64 only.

My tool doesn't scan anything, so I excluded it:

implementation(project(":sdk:client")) {
    exclude(group = "com.google.mlkit")
    exclude(group = "com.google.android.gms", module = "play-services-mlkit-barcode-scanning")
}

That took the APK from 29MB to 8.5MB and native libs from 19.51MB to 233KB. I
went through every screen on a debug build and again on a minified release
build — no missing classes, no crashes.

Two questions. Is excluding it reasonable, or am I working around something I
shouldn't be? And would it make sense for the scanner to be opt-in, so tools
that don't scan don't carry the model?

Originally posted by @jedbridges in #177

[dupontgu]

Ideally we can dynamically add dependencies via a tools lighttool.toml file vs. having devs declare them in the build.gradle. We should also strip out unused deps from the core (The QR stuff being built into the ui module is a good example). Alternatively, break out more specific features like QR into their own submodules.