fix ugly media previews in conversation tab (#1762)

app/src/main/res/layout/item_conversation.xml

@@ -199,6 +199,7 @@
         android:layout_width="0dp"
         android:layout_height="wrap_content"
         android:layout_marginTop="@dimen/status_media_preview_margin_top"
+        android:background="@drawable/media_preview_outline"
         app:layout_constraintEnd_toEndOf="parent"
         app:layout_constraintStart_toStartOf="@id/status_display_name"
         app:layout_constraintTop_toBottomOf="@id/button_toggle_content"

app/src/main/res/layout/item_media_preview.xml

@@ -113,8 +113,8 @@
 
         <TextView
             android:id="@+id/status_sensitive_media_warning"
-            android:layout_width="0dp"
-            android:layout_height="0dp"
+			android:layout_width="wrap_content"
+			android:layout_height="wrap_content"
             android:background="@drawable/media_warning_bg"
             android:gravity="center"
             android:importantForAccessibility="no"