[Thread] clear calling identity before invoking app-supplied callback

Per API council feedback and API guideline at
go/android-api-guidelines#provide-executor, it should call
Binder.clearCallingIdentity() before invoking the app-supplied callbacks
so that the app doesn't use the system-server's identity for permission
check in the callback.

Bug: 309952666
Test: atest ThreadNetworkUnitTests
Change-Id: I3cfbce29c0a516506bb76a44b151a583c8477806
diff --git a/thread/tests/unit/AndroidTest.xml b/thread/tests/unit/AndroidTest.xml
index 663ff74..597c6a8 100644
--- a/thread/tests/unit/AndroidTest.xml
+++ b/thread/tests/unit/AndroidTest.xml
@@ -27,6 +27,7 @@
 
     <test class="com.android.tradefed.testtype.AndroidJUnitTest" >
         <option name="package" value="android.net.thread.unittests" />
+        <option name="hidden-api-checks" value="false"/>
         <!-- Ignores tests introduced by guava-android-testlib -->
         <option name="exclude-annotation" value="org.junit.Ignore"/>
     </test>