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
diff --git a/core/java/android/net/NetworkCapabilities.java b/core/java/android/net/NetworkCapabilities.java
index f43385d..8ebd139 100644
--- a/core/java/android/net/NetworkCapabilities.java
+++ b/core/java/android/net/NetworkCapabilities.java
@@ -1283,6 +1283,7 @@
      * Gets the SSID of this network, or null if none or unknown.
      * @hide
      */
+    @SystemApi
     public @Nullable String getSSID() {
         return mSSID;
     }