[CTS] Fix tearDown() in multi-user tests for assumption failures

tearDown() executes even if assumption failures occur in setUp(). We
need to check if state is defined from setUp(), otherwise we throw
errors if the test device fails an assumption (like multi-user support).

Bug: 126388561
Test: atest CtsBackupHostTestCases (with and without multi-user
support).

Change-Id: I388af4daa33e4fa620f8042be94aad515ca500ed
6 files changed