Fix release build: add androidx.window R8 keep rules, wire proguard-rules.pro

This Flutter SDK's Gradle plugin enables R8 release minification by default (FlutterPluginUtils.shouldShrinkResources() returns true), so release builds run minifyReleaseWithR8. R8 then fails on the optional androidx.window.extensions/sidecar classes - compile-time-only stubs referenced by the transitive androidx.window dependency and provided by the OEM at runtime.

- proguard-rules.pro: -dontwarn androidx.window.extensions/sidecar (per R8's own missing_rules.txt) - build.gradle.kts: make isMinifyEnabled/isShrinkResources explicit and add proguardFiles so proguard-rules.pro is actually applied (it was never referenced before, leaving its rules dormant)

android/app/build.gradle.kts

@@ -77,6 +77,12 @@ android {
 
     buildTypes {
         getByName("release") {
+            isMinifyEnabled = true
+            isShrinkResources = true
+            proguardFiles(
+                getDefaultProguardFile("proguard-android-optimize.txt"),
+                "proguard-rules.pro",
+            )
             val releaseSigningConfig = signingConfigs.getByName("release")
             signingConfig = if (keystorePropertiesExists && releaseSigningConfig.storeFile != null) {
                 releaseSigningConfig

android/app/proguard-rules.pro

@@ -29,4 +29,15 @@
 -keep,allowobfuscation,allowshrinking class com.google.gson.reflect.TypeToken
 -keep,allowobfuscation,allowshrinking class * extends com.google.gson.reflect.TypeToken
 
-##---------------End: proguard configuration for Gson  ----------
+##---------------End: proguard configuration for Gson  ----------
+
+##---------------Begin: androidx.window  ----------
+# androidx.window (pulled in transitively, e.g. via the Flutter embedding / WebView)
+# references optional OEM-provided window-management extension and sidecar classes
+# reflectively. They are not present at compile time, so R8 flags them as missing.
+# Suppressing is safe: the classes are supplied by the device at runtime (or absent,
+# in which case the library gracefully falls back). See missing_rules.txt generated by
+# R8 at build/app/outputs/mapping/normalRelease/missing_rules.txt.
+-dontwarn androidx.window.extensions.**
+-dontwarn androidx.window.sidecar.**
+##---------------End: androidx.window  ----------