update proguard rules for androidx library, fixes #810 (#811)

opentasks/proguard.cfg

@@ -38,11 +38,7 @@
 
 # Keep our own classes to keep stack traces readable. 
 -keep class org.dmfs.** {*;}
-
-# Allow obfuscation of android.support.v7.internal.view.menu.**
-# to avoid problem on Samsung 4.2.2 devices with appcompat v21
-# see https://code.google.com/p/android/issues/detail?id=78377
--keep class !android.support.v7.internal.view.menu.**,android.support.** {*;}
+-keep class androidx.** {*;}
 
 # this is required to keep the Annotations for retention-magic and to keep the line numbers n stack traces
 -keepattributes *Annotation*,SourceFile,LineNumberTable
@@ -77,4 +73,11 @@
     java.lang.String TAG;
     @org.dmfs.android.retentionmagic.annotations.* <fields>;
     private long mId;
+}
+
+# same for support library Fragments
+-keepclassmembers class * extends androidx.fragment.app.Fragment {
+    java.lang.String TAG;
+    @org.dmfs.android.retentionmagic.annotations.* <fields>;
+    private long mId;
 }