Fix Keep rules for MotionLayout

We need to add keep rules for Key so the scene KeyFrames can be
processed properly. Without this, all the alpha elements in the
KeyFrames are not read.

Test: mp sysuig
Bug: 215584502
Change-Id: I960725d67bb80a0c0e2f76f434ca975dfeb08b76

packages/SystemUI/proguard.flags

@@ -11,6 +11,11 @@
 -keep class * extends com.android.systemui.CoreStartable
 -keep class * implements com.android.systemui.CoreStartable$Injector
 
+# Needed for builds to properly initialize KeyFrames from xml scene
+-keepclassmembers class * extends androidx.constraintlayout.motion.widget.Key {
+  public <init>();
+}
+
 -keepclasseswithmembers class * {
     public <init>(android.content.Context, android.util.AttributeSet);
 }