Improve CommunalBackupHelperTest stability

Sometimes setUp() can fail, and tearDown() is still invoked, leading to
an exception that lateinit variables may not have been initialized yet.
This change fixes that by using onTearDown() next to the initialation of
lateinit and only clean up if they have been initialized.

Test: atest CommunalBackupHelperTest
Bug: 379255212
Flag: EXEMPT improve unit test only
Change-Id: I3205dba6abcc7b91d145c5f5432b59992515c60b