Fix the flaky AccessibilityMagnificationTest#testListener

The listner might invoked twice because we use vsync to
change the magnification spec. When the animation is running,
the posted tasks might executed after animation ended twice.

To fix, we update the source bounds only when it is indeed changed,
which also could reduce the redudant mirroing content operation when
the request is sent consecutively in a short time.

Bug: 214785717
Test: atest AccesibilityMagnificationTest \
      --retry-strategy ITERATIONS --max-testcase-run-count 20
Change-Id: I68962b189d8f39f587b717e8125a224e0f67a7e7