Add RootTargetPreparer for CtsInitTestCases and CtsFsMgrTestCases

Some of the test cases in CtsInitTestCases and CtsFsMgrTestCases require
root. This CL makes it possible to run such tests cases on userdebug
builds, which in turn means that they will be run on presubmit.

New option was implemented in
https://android-review.googlesource.com/c/platform/tools/tradefederation/+/1283834

Test: atest CtsInitTestCases
Test: atest CtsFsMgrTestCases
Change-Id: I3d29789ddd7ac549e40ac193c58d986670c49285