Fix TestDroiddoc's reliance on absolute paths

TestDroiddoc compares the value returned by OutputFiles(""), which will
usually be absolute paths including the temporary buildDir, against
paths returned from TestingBuildParam.RelativeToTop(), which does not
currently change the Path contents and so will include absolute
temporary paths. However, a follow up change to this will make the
TestingBuildParam.RelativeToTop() also change the Path contents at
which point this test would be comparing relative to absolute paths.

So, this change makes sure that they are all converted to relative to
top paths before comparison.

Bug: 183650682
Test: m droid
Change-Id: Ia4478f527af27a920945f5849525e5031cc5b8b6