Apply several fixes for beta feedback.

* Remove overscroll from avatar picker recyclers.
* Center crop wallpaper previews.
* If no media thumb exists, return bubble projection instead.

app/src/main/java/org/thoughtcrime/securesms/util/views/NullableStub.java

@@ -0,0 +1,40 @@
+package org.thoughtcrime.securesms.util.views;
+
+
+import android.view.ViewStub;
+
+import androidx.annotation.NonNull;
+import androidx.annotation.Nullable;
+
+import java.util.Objects;
+
+public class NullableStub<T> {
+
+  private ViewStub viewStub;
+  private T view;
+
+  public NullableStub(@Nullable ViewStub viewStub) {
+    this.viewStub = viewStub;
+  }
+
+  private @Nullable T get() {
+    if (viewStub != null && view == null) {
+      view = (T) viewStub.inflate();
+      viewStub = null;
+    }
+
+    return view;
+  }
+
+  public @NonNull T require() {
+    return Objects.requireNonNull(get());
+  }
+
+  public boolean isResolvable() {
+    return viewStub != null || resolved();
+  }
+
+  public boolean resolved() {
+    return view != null;
+  }
+}