Fix Adb root/unroot for encryption test

These steps are causing issue with Gce devices, root
should be done via the ITestDevice API enableAdbRoot.

Bug: 29127775
Change-Id: I0abc2f0d8d173f72ae2c6eb93bbcf58ffe95efd1
diff --git a/hostsidetests/security/src/android/security/cts/EncryptionHostTest.java b/hostsidetests/security/src/android/security/cts/EncryptionHostTest.java
index 5e12143..8d9e576 100644
--- a/hostsidetests/security/src/android/security/cts/EncryptionHostTest.java
+++ b/hostsidetests/security/src/android/security/cts/EncryptionHostTest.java
@@ -41,9 +41,15 @@
         if (!mUserDebug) {
             return;
         }
+        mDevice.enableAdbRoot();
+    }
 
-        mDevice.executeAdbCommand("root");
-        assertTrue(mDevice.isAdbRoot());
+    @Override
+    protected void tearDown() throws Exception {
+        if (mUserDebug) {
+            mDevice.disableAdbRoot();
+        }
+        super.tearDown();
     }
 
     public void testEncrypted() throws DeviceNotAvailableException {