[Thread][API] add API for ephemeral key.

This CL adds the API for starting/stopping the ephemeral key mode, and
subscribing to the ephemeral key state changes.

Ephemeral key mode is started with a limit amount of time to allow
Thread credential sharing.

Bug: 348323500
Test: atest CtsThreadNetworkTestCases
Change-Id: I540c88ec4668291358fbe87a678d8f9d31a67e54
diff --git a/thread/tests/cts/AndroidTest.xml b/thread/tests/cts/AndroidTest.xml
index 34aabe2..e954d3b 100644
--- a/thread/tests/cts/AndroidTest.xml
+++ b/thread/tests/cts/AndroidTest.xml
@@ -56,4 +56,14 @@
         <!-- Ignores tests introduced by guava-android-testlib -->
         <option name="exclude-annotation" value="org.junit.Ignore"/>
     </test>
+
+    <!--
+        This doesn't override a read-only flag, to run the tests locally with `epskc_enabled` flag
+        enabled, set the flag to `is_fixed_read_only: false`. This should be removed after the
+        `epskc_enabled` flag is rolled out.
+    -->
+    <target_preparer class="com.android.tradefed.targetprep.FeatureFlagTargetPreparer">
+        <option name="flag-value"
+                value="thread_network/com.android.net.thread.flags.epskc_enabled=true"/>
+    </target_preparer>
 </configuration>