Only run RebootTest under root

This test requires running test services, which causes test to crash
(and still incorrectly be reported as passing) when running on
non-rooted device.

Ignore-AOSP-First: reboot_test is not in AOSP yet
Bug: 190958734
Test: atest CtsInitTestCases
Change-Id: I3c5c9917d0a787d66272ccf4aefc57e6573841bc