Support minifcation for apps that depend on AGP 8 and integration_test (#127628)

- Set exported proguard rules for consumers of integration_test using AGP 8.0 
- Updated proguard usage in example to test setting custom proguard rules works.

#127388

Documentation for how consumerProguardFiles works. https://developer.android.com/studio/projects/android-library#Considerations

Unknown why we had to use ** instead of matching exactly.

packages/integration_test/android/build.gradle

@@ -41,6 +41,7 @@ android {
     defaultConfig {
         minSdkVersion 19
         testInstrumentationRunner "androidx.test.runner.AndroidJUnitRunner"
+        consumerProguardFiles 'lib-proguard-rules.txt'
     }
 
     dependencies {

packages/integration_test/android/lib-proguard-rules.txt

@@ -0,0 +1,5 @@
+# For some reason org.kxml2.io.KXmlParser and org.kxml2.io.KXmlSerializer
+# are missing and not marked correctly by dependencies.
+# Possibly related to https://github.com/flutter/flutter/issues/56591
+# See https://github.com/flutter/flutter/issues/127388 for more context.
+-dontwarn org.kxml2.io.KXmlParser**,org.kxml2.io.KXmlSerializer**

packages/integration_test/example/android/app/build.gradle

@@ -55,8 +55,7 @@ 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.debug
-            minifyEnabled false
-            shrinkResources false
+            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
         }
     }
 }

packages/integration_test/example/android/app/proguard-rules.pro

@@ -0,0 +1 @@
+# Intentionally empty. Validation is that apps can set their own proguard files.