[STL] Fix predictive back with activity-compose:1.10.0-alpha03

activity-compose:1.10.0-alpha03 introduced a stricter coroutine
cancellation policy. The coroutine is now cancelled immediately when the
PredictiveBackHandler is disabled. Therefore we have to run the
transition on the animation scope instead.

Bug: 376154092
Flag: EXEMPT bugfix
Test: PredictiveBackHandlerTest
Change-Id: I2c9eda8c6c51335c547b10430ecfb34db02752f0