Update flag constant for caption on shell

System properties under persist.debug namespace can't be modified by
SystemUI. aosp/I5808bf92dbba37e9e6da5559f8e0a5fdac016bf3 introduced a
new namespace, persist.wm.debug that can be used for system properties
that can be overridden from SystemUI.
Move captions_on_shell to this namespace so we can update this flag from
SystemUI to simplify enabling it on devices.

Bug: 241464028
Test: build succeeds
Change-Id: Ifcf54b6dfa70ca52dfb51fb2f7f2e67d0568566a