Fix color application to back button on bouncer

Same as with http://ag/34810040, the text color should actually be
applied to the outer button element and not the inner text element.
This allows for the button to have the proper colors when it is
disabled (it's currently always enabled though).

Bug: 418724618
Flag: com.android.systemui.scene_container
Test: atest BouncerOverlayContentViewModelTest
Test: atest BouncerContentComposeTest
Change-Id: Ib9d61044e753aa896f838e13015c8baf2f77a3fe