fix(magnification button): delay showing magnification button to prevent...

fix(magnification button): delay showing magnification button to prevent button from detecting the touch expectedly

When magnification mode switch button is hiding, finger down event at the button area will first trigger MagnificationController#onUserInteractionStart to show the button, then the down event will be detected by the button too. This causes tapping on the hiding button area will show both the button and the settings panel. Therefore, when MagnificationController calls the systemui to show the button, we delay the showButton at systemui side, so the button will not show immediately then detects the touch event unexpectedly.

Bug: 338259519
Flag: ACONFIG com.android.systemui.delay_show_magnification_button DEVELOPMENT
Test: manually flip the flag
      atest IMagnificationConnectionTest
Change-Id: I7852633914ab93126efc92199a904ef784d775af