[flexiglass] Adds predictive_back_sysui as a flag dependency.

Flexiglass now requires this flag to be on.

Fix: 332741685
Test: with cl/621645064, made sure that turning flexiglass on using the script at
go/flexiglass-script also makes back work on the bouncer and QS scenes
in flexiglass
Flag: N/A

Change-Id: Ic2a61032bc28b9434222a3604a46b2228b317765