[Predictive Back] Gate feature under experiment flag

As predictive back will be launched for U-QPR, we should gate it to avoid it leaked to U launch.

Test: turn on/off ENABLE_BACK_SWIPE_LAUNCHER_ANIMATION flag and verify predicitive back is turned on/off
Bug: 264920023
Change-Id: Ieee14ca9d49f052f183d8734d62443b890210544
1 file changed