[testing] Bump R8 to 9.1.43

[jonathanpeppers]

⚠️ For testing only — not intended to merge as-is.

Bumps the R8/D8 shipped in .NET for Android from 9.1.31 → 9.1.43.

Why

R8 9.1.31 throws a NullPointerException in AndroidApiLevelCompute.of() when invoked with --no-tree-shaking. Google's R8 team fixed this in 9.1.43.

• https://issuetracker.google.com/issues/524775142
• Crash in r8 when building with .NET 11 #11672

How

9.1.43 is a stable tagged R8 release, but it has not yet been published to maven.google.com (which still tops out at 9.1.31). It is published on the R8 team's own release feed at https://storage.googleapis.com/r8-releases/raw, which is the channel Google's R8 docs point users at for exactly this case — see Replacing R8 in AGP.

src/r8/build.gradle now:

1. Adds https://storage.googleapis.com/r8-releases/raw as a maven repository (after google() and mavenCentral(), so Google Maven is still preferred when a version is available there).
2. Bumps the dependency to com.android.tools:r8:9.1.43.

Verification

Built locally via the existing gradle build that backs src/r8/r8.csproj:

> Task :jar
BUILD SUCCESSFUL in 9s

And confirmed the produced fat-jar reports the right version:

$ java -cp src/r8/build/libs/r8.jar com.android.tools.r8.R8 --version
R8 9.1.43 (build 97a09ffd53fb2082ad69827578fed444da540740 from go/r8bot ...)
$ java -cp src/r8/build/libs/r8.jar com.android.tools.r8.D8 --version
D8 9.1.43 (build 97a09ffd53fb2082ad69827578fed444da540740 from go/r8bot ...)