Fix incorrect remove animation switch status after turned off Talkback

Root cause: The ag/3373822 introduce the comparison by string since 2017, but it would conflict with AccessibilityService#setAnimationScale(float) which is  added by API level 33.
Solution: Use float comparison instead and observe the key changed to update status if the setting page is on foreground.

Bug: 244687190
Test: atest DisableAnimationsPreferenceControllerTest
Change-Id: I59b7edb70a27af4276eccd6f19d2ef46ba30b604