Move up verify phone button (#1708)

Author: samtstern

auth/src/main/res/layout/fui_phone_layout.xml

@@ -48,7 +48,6 @@
                 style="@style/FirebaseUI.VerifyPhoneButton"
                 android:layout_marginTop="@dimen/fui_field_padding_vert"
                 android:text="@string/fui_verify_phone_number"
-                app:layout_constraintBottom_toBottomOf="parent"
                 app:layout_constraintStart_toStartOf="parent"
                 app:layout_constraintTop_toBottomOf="@+id/phone_layout" />