Reinstate Proguard/R8 (#5134)

* Reinstate Proguard/R8

According to APK analyzer
Before: 18044 classes with 113387 methods in 3 dex files
After: 12604 classes with 68010 methods in 2 dex files
APK download size is reduced by 2.8 MB

* Add debug build type without R8

app/proguard-android-optimize.txt

@@ -0,0 +1,34 @@
+-allowaccessmodification
+-dontusemixedcaseclassnames
+-verbose
+
+-keepattributes *Annotation*
+
+-keepclasseswithmembernames,includedescriptorclasses class * {
+    native <methods>;
+}
+
+-keepclassmembers enum * {
+    public static **[] values();
+    public static ** valueOf(java.lang.String);
+}
+
+-keepclassmembers class * implements android.os.Parcelable {
+  public static final ** CREATOR;
+}
+
+-keep class androidx.annotation.Keep
+
+-keep @androidx.annotation.Keep class * {*;}
+
+-keepclasseswithmembers class * {
+    @androidx.annotation.Keep <methods>;
+}
+
+-keepclasseswithmembers class * {
+    @androidx.annotation.Keep <fields>;
+}
+
+-keepclasseswithmembers class * {
+    @androidx.annotation.Keep <init>(...);
+}