Add predictive back aconfig flag

Bug: 309545085
Flag: ACONFIG com.android.window.flags.predictive_back_system_animations DISABLED
Test: atest BackAnimationPreferenceControllerTest; Manual, i.e. building and verifying correct behaviour for different flag configurations
Change-Id: I3f478dc23bbfeb1f13cae64f3d04b3210841731d