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

telephony/java/android/telephony/SubscriptionInfo.java

@@ -662,8 +662,16 @@ public class SubscriptionInfo implements Parcelable {
     }
 
     /**
-     * @return the card string of the SIM card which contains the subscription. The card string is
-     * the ICCID for UICCs or the EID for eUICCs.
+     * Returns the card string if the calling app has been granted the READ_PRIVILEGED_PHONE_STATE
+     * permission, has carrier privileges (see {@link TelephonyManager#hasCarrierPrivileges}), or
+     * is a device owner or profile owner on an organization owned device that has been granted the
+     * READ_PHONE_STATE permission. The profile owner is an app that owns a managed profile on the
+     * device; for more details see <a href="https://developer.android.com/work/managed-profiles">
+     * Work profiles</a>.
+     *
+     * @return the card string of the SIM card which contains the subscription or an empty string
+     * if these requirements are not met. The card string is the ICCID for UICCs or the EID for
+     * eUICCs.
      * @hide
      * //TODO rename usages in LPA: UiccSlotUtil.java, UiccSlotsManager.java, UiccSlotInfoTest.java
      */
@@ -671,6 +679,13 @@ public class SubscriptionInfo implements Parcelable {
         return this.mCardString;
     }
 
+    /**
+     * @hide
+     */
+    public void clearCardString() {
+        this.mCardString = "";
+    }
+
     /**
      * Returns the card ID of the SIM card which contains the subscription (see
      * {@link UiccCardInfo#getCardId()}.