Allow statsd_testdrive to work in root

LocalDrive needs to handle the situation whereby the caller does 'adb
root' in between calls, and therefore runs everything as shell.

TestDrive is a single-call, so it can just run as the caller. That's
what it used to do, but a recent refactoring made some of its calls via
shell, causes an error. This fixes that.

Test: manual confirmation that it worked for both shell and root
Change-Id: I5e31cdd59d61290a480cb6fae107170616daabc0