Use correct call to get Android user ID

Rather than trying to store the active user ID, use
multiuser_get_user_id to pull the actual ID for the caller. This will
now behave correctly when called by background processes.

Test: CtsKeystoreTestCases, from both owner and guest users
Bug: 113280004
Change-Id: I73fd2ef2ac4b1ceb7ec3792761e827d801d0d905
diff --git a/keystore/key_store_service.h b/keystore/key_store_service.h
index 0056342..8d3f1f2 100644
--- a/keystore/key_store_service.h
+++ b/keystore/key_store_service.h
@@ -39,7 +39,7 @@
   public:
     explicit KeyStoreService(KeyStore* keyStore)
         : mKeyStore(keyStore), mOperationMap(this),
-          mConfirmationManager(new ConfirmationManager(this)), mActiveUserId(0) {}
+          mConfirmationManager(new ConfirmationManager(this)) {}
     virtual ~KeyStoreService() = default;
 
     void binderDied(const android::wp<android::IBinder>& who);
@@ -300,7 +300,6 @@
     android::sp<ConfirmationManager> mConfirmationManager;
     keystore::AuthTokenTable mAuthTokenTable;
     KeystoreKeymasterEnforcement enforcement_policy;
-    int32_t mActiveUserId;
 };
 
 };  // namespace keystore