Add NetworkCapabilities#getSSID to SystemApi

setSSID was added to the API, so it makes sense to have getSSID be part
of the API as well.

Test: m
Bug: 139268426
Change-Id: I6fa9c513ce4fb8ce1e3530776ce8fd5b2e77055e

core/java/android/net/NetworkCapabilities.java

@@ -1283,6 +1283,7 @@ public final class NetworkCapabilities implements Parcelable {
      * Gets the SSID of this network, or null if none or unknown.
      * @hide
      */
+    @SystemApi
     public @Nullable String getSSID() {
         return mSSID;
     }