Update proguard rules to a more modern setup

Took 8 seconds

android/app/build.gradle

@@ -69,8 +69,9 @@ android {
         release {
             signingConfig signingConfigs.release
             minifyEnabled true
+            shrinkResources true
 
-            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
+            proguardFiles getDefaultProguardFile('proguard-android-optimize.txt'), 'proguard-rules.pro'
         }
     }
 }

android/app/proguard-rules.pro

@@ -6,4 +6,36 @@
 -keep class io.flutter.view.**  { *; }
 -keep class io.flutter.**  { *; }
 -keep class io.flutter.plugins.**  { *; }
--dontwarn io.flutter.embedding.**
+-dontwarn io.flutter.embedding.**
+
+##---------------Begin: proguard configuration for Gson (Needed for flutter_local_notifications)  ----------
+# Gson uses generic type information stored in a class file when working with fields. Proguard
+# removes such information by default, so configure it to keep all of it.
+-keepattributes Signature
+
+# For using GSON @Expose annotation
+-keepattributes *Annotation*
+
+# Gson specific classes
+-dontwarn sun.misc.**
+
+# Application classes that will be serialized/deserialized over Gson
+-keep class com.google.gson.examples.android.model.** { <fields>; }
+
+# Prevent proguard from stripping interface information from TypeAdapter, TypeAdapterFactory,
+# JsonSerializer, JsonDeserializer instances (so they can be used in @JsonAdapter)
+-keep class * extends com.google.gson.TypeAdapter
+-keep class * implements com.google.gson.TypeAdapterFactory
+-keep class * implements com.google.gson.JsonSerializer
+-keep class * implements com.google.gson.JsonDeserializer
+
+# Prevent R8 from leaving Data object members always null
+-keepclassmembers,allowobfuscation class * {
+  @com.google.gson.annotations.SerializedName <fields>;
+}
+
+# Retain generic signatures of TypeToken and its subclasses with R8 version 3.0 and higher.
+-keep,allowobfuscation,allowshrinking class com.google.gson.reflect.TypeToken
+-keep,allowobfuscation,allowshrinking class * extends com.google.gson.reflect.TypeToken
+
+##---------------End: proguard configuration for Gson (Needed for flutter_local_notifications)  ----------

android/gradle.properties

@@ -2,3 +2,4 @@ org.gradle.jvmargs=-Xmx1536M
 android.enableR8=true
 android.useAndroidX=true
 android.enableJetifier=true
+android.enableR8.fullMode=true