Fix MixedDeviceOwnerTest#testAccountManagement_disabledAddAccount

SetPolicyActivity posts finish() in onResume, and this causes
timeout of the shell command that was used to launch activity.
This CL removes call to finish() in onResume, fixing timeout problem.
Without finish() call, single instance of activity can be used
in multiple tests. This case is handled by overloaded onNewIntent()

Bug: 150865588
Test: arcts-tradefed run arcts
Test: cts-tradefed run cts -m CtsDevicePolicyManagerTestCases
Change-Id: Ifea988720e3b50161e2ec4cba31a1859fad72aff