Stop ignoring WindowMagnificationAnimationControllerTest.

@Ignore was added in
commit 5830f078

However, @Ignore-ing a whole test class seems to cause a strange
TEST_MAPPING error: "Runner reported an invalid method 'null'"

Testing locally this test is passing on my device, so lets try to
run it in postsubmit instead of skipping it altogether. Moderate
flakiness is allowed in postsubmit.
- If it passes reliably in postsubmit we can move it to presubmit.
- If it is indeed flaky in postsubmit we can use @FlakyTest to
  exclude just this test from presubmit, and then we can move the
  other non-flaky SysUI a11y tests to presubmit.

Bug: 280087622
Bug: 278920911
Test: atest WindowMagnificationAnimationControllerTest
Change-Id: Iba9f4d0f20be9e9a3cea5605a8c6f46da33933af