Refactor FilePathAccessTest

Move most of the helper functions out of FilePathAccessTest so that
it would be easier to reuse them for other tests and add additional
utilities to the same test.

Add documentation to helper methods.

This is a step to make it easier to add tests for metadata redaction.

Test: atest FuseDaemonHostTest
Bug: 145795132
Change-Id: Ie8961b9448795599dc4a2f9d91003bc8fd3d473d
6 files changed