Add SpaEnvironment.isSpaExpressiveEnabled

- Avoid inaccessible code in spa.
- Each client can decide whether to enable this by themselves.

Bug: 386013400
Flag: com.android.settingslib.widget.theme.flags.is_expressive_design_enabled
Test: visual - expressive can enabled for spa in Settings
Change-Id: Ieed10d7bbabb17fcfb977aae9aecd3a33e6ee65d