Support demo mode and demo users

Bug: 62712426
Test: run cts -m CtsDevicePolicyManagerTestCases -t \
com.android.cts.devicepolicy.DeviceOwnerTest#testCreateAndManageEphemeralUser
and
run cts -m CtsDevicePolicyManagerTestCases -t \
com.android.cts.devicepolicy.DeviceOwnerTest# \
testCreateAndManageEphemeralUserFailsWithoutSplitSystemUser
and
runtest -c com.android.server.devicepolicy.DevicePolicyManagerTest \
frameworks-services
Change-Id: I77a71a994fe0f4f1f8c5df7c4ccf493aafa8fefe