Use current user isntead of system user if no --user parameter is passed

This is relevant when running cts tests in instant mode.
Certain tests were failing in non-system users in instant mode, which is particularlly consquential to auto.
THis fix will be cherry-picked back to their repo
Test: CTSAccessibility*, in both instant/non-instant, and with both system and secondary users.
Bug:146218533

Change-Id: I89937bd552d372adc5d96c51b24d1a2bb4a034e3