Experience upgrade splash screen.

Behaves similarly to the DatabaseUpgradeActivity. You have a
static list of ExperienceUpgrade models that include a "trigger"
version, where when a user upgrades through it a notification
will appear, and there will be a splash explanation screen.

Right now the splash screens are basic and not too configurable,
but that can be reworked as upgrades demand.

Closes #4151

src/org/thoughtcrime/securesms/util/ViewUtil.java

@@ -16,7 +16,7 @@
  */
 package org.thoughtcrime.securesms.util;
 
-import android.content.Context;
+import android.app.Activity;
 import android.graphics.drawable.Drawable;
 import android.os.Build.VERSION;
 import android.os.Build.VERSION_CODES;
@@ -72,6 +72,11 @@ public class ViewUtil {
     return (T) parent.findViewById(resId);
   }
 
+  @SuppressWarnings("unchecked")
+  public static <T extends View> T findById(@NonNull Activity parent, @IdRes int resId) {
+    return (T) parent.findViewById(resId);
+  }
+
   private static Animation getAlphaAnimation(float from, float to, int duration) {
     final Animation anim = new AlphaAnimation(from, to);
     anim.setInterpolator(new FastOutSlowInInterpolator());