Replace your recyclerView with,
<android.support.v7.widget.RecyclerView android:id="@+id/recyclerView" app:layout_behavior="@string/appbar_scrolling_view_behavior" android:layout_width="match_parent" android:layout_height="wrap_content" />
here,
app:layout_behavior="@string/appbar_scrolling_view_behavior"
will manage the rest of things.
One more thing, no need to put your recyclerView inside NestedScrollView

Replace your recyclerView with,
<android.support.v7.widget.RecyclerView android:id="@+id/conversation" app:layout_behavior="@string/appbar_scrolling_view_behavior" android:layout_width="match_parent" android:layout_height="wrap_content" /> here,
app:layout_behavior="@string/appbar_scrolling_view_behavior" will manage the rest of things.
One more thing, no need to put your recyclerView inside NestedScrollView