Added flag and tests for SubscriptionPlan changes

Adding a flag and supporting tests for changes to SubscriptionPlan to
provide a subscription status and the plan's end date if available.

Flag: com.android.internal.telephony.flags.subscription_plan_allow_status_and_end_date

Test: atest phone FrameworksTelephonyTests:SubscriptionPlanTest
Bug: 357272015
Change-Id: I25b70907eda6b3bcbf34f786ed86d64432c7331f