Introduce a flag for new back affordance

Also, turn it on by default.

This allows the new back indicator to be tested seperately from
predictive back animations.

Test: adb shell broadcast -a com.android.systemui.action.SET_FLAG \
        --ei id 1203 --ez value 1
Bug: 217579545
Change-Id: I2f3a3b1413e25bcac04cd7ad5b288565478101b9