Further Clarify Scope of KEY_CARRIER_NAME_OVERRIDE

There is still confusion about the scope of the
KEY_CARRIER_NAME_OVERRIDE. This CL further clarifies
that the carrier, for purposes of this string, is the
service provider. Thus, the CARRIER_NAME_OVERRIDE will
still be subject to SPN display rules.

Note: to use this override in scenarios such as roaming
where the display rules may not choose the name of the
home operator (and thus not choose this override), a
service provider may choose to use other flags such as
KEY_FORCE_HOME_NETWORK_BOOL and
KEY_SPN_DISPLAY_RULE_USE_ROAMING_FROM_SERVICE_STATE_BOOL
to achieve the desired display behavior.

Bug: 77461054
Test: none (docstring-only change)
Merged-In: Id866df7ef9ce593de1f9d31d2556af4bfc766a8e
Change-Id: Id866df7ef9ce593de1f9d31d2556af4bfc766a8e
(cherry picked from commit 178020a2)