fix(fullscreen magnification): add flag for drawing fullscreen magnification border in SystemUI

Add the flag for:
1. Wrapping the contents in methods that are related to border drawing
2. Avoid creating the MagnifiedViewport that is used for managing UI-related components to save memory

Bug: 291891390
Test: manually enable/disable the flag to check if the original orange border exists
      atest WindowManagerServiceTests#testDrawMagnifiedViewport
      atest frameworks/base/services/tests/servicestests/src/com/android/server/accessibility/magnification
Flag: ACONGIF com.android.window.flags.magnification_always_draw_fullscreen_border DEVELOPMENT

Change-Id: I7c9a61afd4dfee5a64219a8f3e908b9512f68b6d