Fix back button circle style in expressive theme

According to BC25 style guideline, there should be circle around the
back button in setting screens. SC and PC didn't have this expressive
style because the action bar was returning null in
CollapsingToolbarBaseFragment. In this CL, change this behavior to be
returning the action bar object when expressive theme is enabled.

Test: manual and CtsPermissionUiTestCases
Flag: com.android.settingslib.widget.theme.flags.is_expressive_design_enabled
Bug: 419658752
Change-Id: Id9279cbfbde4b873b5a349f8ebe2c375e9f4aa16