Add Path.RelativeToTop

Makes Path.RelativeToTop the source of truth as to what `relative to
top` means.

Uses it in PathRelativeToTop so that it will be tested by the existing
tests that use AssertPathRelativeToTop.

Also adds RelativeToTop() to WritablePaths and Paths.

Bug: 183650682
Test: m droid
Change-Id: I88dc56afd1314e51b4e41795a2448cab4ce8a899