Fix test failure

Fix test failure by adding an EnableFlag annotation. The test was
failing because when the notification_content_alpha_optimization flag is
disabled, there's no call to getAlpha() when the verification requires
one.

Bug: 355132257
Bug: 292024656
Change-Id: If43249218cb7bc103e412832ade38a1246897673
Fix: 359933016
Test: atest NotificationSwipeHelperTest
Flag: com.android.systemui.notification_content_alpha_optimization