Update carrier privilege check in enforceSubscriptionPlanAccess

Remove isEmbedded() check, since this can be called on both PSIM and
ESIM. Replace SM#canManageSubscription with TM#hasCarrierPrivileges,
since it is a superset of the checks done in SM#canManageSubscription
and applies to both PSIMs as well as ESIMS. Updated the documentation
for the two APIs to clarify the differences.

Test: atest CtsTelephonyTestCases (SubscriptionManagerTest)
Fix: 180451446
Change-Id: Icb4e2c619dacaea9e74cedea31865ec1eb689dce