SubscriptionPlan: move out of telephony

Hidden APIs of SubscriptionPlan are used in system server, not
telephopny. So it makes more sense for SubscriptionPlan to be in
core instead of telephopny.

Bug: 145814152
Test: make
Change-Id: I3bfb0ec699f46b0cc34c3bc2e13bfb242bc295bf
diff --git a/telephony/java/android/telephony/SubscriptionPlan.aidl b/core/java/android/telephony/SubscriptionPlan.aidl
similarity index 100%
rename from telephony/java/android/telephony/SubscriptionPlan.aidl
rename to core/java/android/telephony/SubscriptionPlan.aidl