sysui: Have date/weather flag as sysprop flag.

  The change allows the flag to be shared across different
  processes without overhead of content provider lookups.

  Flag can be set via
     adb shell setprop persist.sysui.ss.dw_decoupled
     <bool>

Bug: 261757708
Test: manual
Change-Id: Iec55f51ead4c1001b0f761bfd4e2e6e974abec0e