Hide back button behind flag

The button has a weird bug (b/432062209), which I can't seem to solve.
Hiding this behind a flag until we find a solution.

Bug: 432062209
Test: atest BouncerContentComposeTest
Flag: com.android.systemui.back_button_on_bouncer

Change-Id: I339cd9f76b155db3491e8f8d77628935a7a97f40