Fix @Nullability mismatch with PipTransition

Most of the codebase injecting `PipTransition` expects a non-null object. Trying
to inject a null will outright crash. Moreover, nullability mismatch breaks
sysui-studio builds which actually properly typecheck this nullability.

This CL provides legacy transition until non-null pip2.PipTransition is available.

Bug: 293844330
Test: verified the sysui-studio build now builds and runs
Change-Id: I330ad86a70ddfbf0d08bd6d2ed408d6856ca3162