fix(android): keep the JNI callback names through R8

Release builds aborted on the first callback of every test:

  NoSuchMethodError: no non-static method "Lp0;.onOutput(Ljava/lang/String;)V"

R8 had renamed MainThreadCallback to p0 along with its methods, so
GetMethodID(env, cls, "onOutput", ...) in iperf_jni.c resolved nothing. The
names are a contract between C and Kotlin that the minifier cannot see.

Adds keep rules for IperfRunner, the IperfCallback interface and — the part
that matters — everything implementing it, since the lookup happens on the
concrete object's class rather than on the interface. A general rule keeps any
class declaring a native method, so a future one is covered without a new rule.

Debug builds do not minify, which is why every on-device check so far passed
while the shipping artifact was broken. Found by running the release APK
during release preparation.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

android/app/build.gradle.kts

@@ -37,6 +37,13 @@ android {
             // TODO: Add your own signing config for the release build.
             // Signing with the debug keys for now, so `flutter run --release` works.
             signingConfig = signingConfigs.getByName("debug")
+
+            // proguard-rules.pro keeps the names JNI resolves at runtime.
+            // Without it a release build aborts on the first callback.
+            proguardFiles(
+                getDefaultProguardFile("proguard-android-optimize.txt"),
+                "proguard-rules.pro",
+            )
         }
     }