Add --top_level_phony to catch more real-to-phony problems

As more phony rules move either to the packaging kati run, or soong, the
existing real-to-phony check failed to notice that they were phony
rules.

So I added --top_level_phony to Make that assumes that all leaf nodes
(source files, or nodes generated by another ninja file generator) that
do not have a '/' in them are phony targets.

The existing phony-looks-real check in Kati should prevent these from
being real generated files, and our source tree shouldn't have any
source files used at the root level.

Test: build_test on downstream branches
Change-Id: I77cff84b536c6cd6402307a9aa5a9c273e72c71f