Add test for getStartUser(), getEndUser() and createForUser()

- Follow-up commit will modify getStartUser() and getEndUser(),
to make sure the result is consistent, add test before changing
getStartUser() and getEndUser().
- Add a test for createForUser() to ensure the last uid of
the previous user is smaller than the first uid of the next user.
- Move this test file to tests/net/common, so that it can be run
in CTS.

Bug: 170598012
Test: atest FrameworksNetTests:UidRangeTest
      CtsNetTestCasesLatestSdk:UidRangeTest
      Also run above tests on Q and R devices.
Change-Id: I8b146aa14b50911e50a39f266e8d8dc17df853b2