Make the DPM account strings system-APIs.

Bug: 33670246
Test: After making the CTS change in the same topic:
 cts-tradefed run cts --skip-device-info --skip-preconditions --skip-system-status-check com.android.compatibility.common.tradefed.targetprep.NetworkConnectivityChecker -a armeabi-v7a -l INFO -m CtsDevicePolicyManagerTestCases -t com.android.cts.devicepolicy.AccountCheckHostSideTest

Change-Id: Icefb6aec04a5ec805313e027424e99bdbf769e67