[XPD] Use DrawableStateLayout background

For the following Preferences:

- Preference
- SelectorWithWidgetPreference
- TwoTargetPreference

...make the layout's root element a DrawableStateLayout, and set its
'background' attribute to "@drawable/settingslib_round_background".

This should have no visual impact, and only has effect when expressive
design is enabled. But it changes the source of the Preference's
background image (instead of ?selectableItemBackground, it will be
@drawable/settingslib_round_background_stateful). (Note that before this
change, the background is currently being set at runtime, in code, to a
fixed background.)

Test: manual
Relnote: N/A
Flag: com.android.settingslib.widget.theme.flags.is_expressive_design_enabled
Bug: 375480009
Change-Id: I9597b867109de218b323f469a4dfbcc30d2564b5