Add API for checking which CA certs were installed by the DO/PO

With this API, the system can determine whether a CA cert was
installed by the user or the user's DO/PO.

Bug: 32692748
Test: unit tests (see DevicePolicyManagerTest.java for invocation)
Test: cts-tradefed run cts-dev --module CtsDevicePolicyManagerTestCases

Change-Id: I3bcae5ac18ec2b110154184fc515df804fd73da6
diff --git a/keystore/java/android/security/IKeyChainService.aidl b/keystore/java/android/security/IKeyChainService.aidl
index b685431..c4bb72c 100644
--- a/keystore/java/android/security/IKeyChainService.aidl
+++ b/keystore/java/android/security/IKeyChainService.aidl
@@ -29,8 +29,8 @@
     byte[] getCertificate(String alias);
     byte[] getCaCertificates(String alias);
 
-    // APIs used by CertInstaller
-    void installCaCertificate(in byte[] caCertificate);
+    // APIs used by CertInstaller and DevicePolicyManager
+    String installCaCertificate(in byte[] caCertificate);
 
     // APIs used by DevicePolicyManager
     boolean installKeyPair(in byte[] privateKey, in byte[] userCert, in byte[] certChain, String alias);