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

api/system-current.txt

@@ -4471,6 +4471,7 @@ package android.net {
   public final class NetworkCapabilities implements android.os.Parcelable {
     method public boolean deduceRestrictedCapability();
+    method @Nullable public String getSSID();
     method @NonNull public int[] getTransportTypes();
     method public boolean satisfiedByNetworkCapabilities(@Nullable android.net.NetworkCapabilities);
     method @NonNull public android.net.NetworkCapabilities setSSID(@Nullable String);

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;
     }