🎢 Add a GN assert preventing android templates in non-default toolchain

build/config/android/config.gni

Index: build/config/android/config.gni
diff --git a/build/config/android/config.gni b/build/config/android/config.gni
index 47c14e63d907fb20eb31451a662e676b333a95f3..3590205c37b8057ede2f66c78cd6eb59b16b5093 100644
--- a/build/config/android/config.gni
+++ b/build/config/android/config.gni
@@ -11,6 +11,13 @@ if (is_android) {
 
   import("//build_overrides/build.gni")
 
+  # Disable all java-related templates in secondary toolchains. Java is not
+  # toolchain-dependent, so a variation on toolchains is likely a mistake.
+  if (enable_java_templates) {
+    assert(current_toolchain == default_toolchain,
+           "Android templates should be used only by the default toolchain.")
+  }
+
   has_chrome_android_internal =
       exec_script("//build/dir_exists.py",
                   [ rebase_path("//clank", root_build_dir) ],