Switch to system user before running incident module.

Module tests features that only work for user 0 (ErrorTest verifies
DropBoxManager which only broadcasts to user 0).
Use target provider to switch to user 0 before the module execution,
and switch back to whatever user we were in after we're done.

bug: 123154921
Test: cts-tradefed run cts-dev -m CtsIncidentHostTestCases
Change-Id: I58c228bbbdfe0c623ee92f5deaa53a80ea79138e