Make CellIdentity#getPlmn() Publicly Accessible

Add support for telephony to access getPlmn().

Bug: 148111627
Test: compilation (trivial)
Change-Id: I5a79e5339a7e4e1351582f199559801069df0c76

telephony/java/android/telephony/CellIdentity.java

@@ -211,7 +211,7 @@ public abstract class CellIdentity implements Parcelable {
     }
 
     /** @hide */
-    protected String getPlmn() {
+    public @Nullable String getPlmn() {
         if (mMccStr == null || mMncStr == null) return null;
         return mMccStr + mMncStr;
     }