Perform magnification viewport drawing outside of WM global lock.

Attempts to fix the deadlock caused by holding onto the WM global lock
too long.

Bug: 276845499
Test: Use magnification, observe no change in behavior
Test: Existing magnification-related tests:
       atest frameworks/base/services/tests/servicestests/ \
         src/com/android/server/accessibility/magnification
       atest frameworks/base/packages/SystemUI/tests/ \
         src/com/android/systemui/accessibility
       atest cts/tests/accessibilityservice/ \
         src/android/accessibilityservice/cts
Change-Id: Id828744c8c5bcb4bdb3be9a11810338614b84b2e