Add method to clear out SubscriptionInfo#getCardString

In Androird 10 access to device identifiers was limited to apps with
the READ_PRIVILEGED_PHONE_STATE permission, carrier privileges, the
READ_DEVICE_IDENTIFIERS appop set to allow, or those that pass a
device / profile owner check. TelephonyManager#getSimSerialNumber
was guarded behind these new access requirements, but the same value
is still accessible as the cardString from the SubscriptionInfo.
While getCardString is a hidden API SubscriptionInfo#toString can
be used to obtain this value.

Bug: 152057778
Test: atest SubscriptionControllerTest
Change-Id: I9b8786a321b1ba79d81b74c3041589f30df8e9c8