Turn on phony checks

--warn_real_to_phony will print a warning any time that a non-PHONY rule
depends on a PHONY rule. Since PHONY rules are always considered dirty
this would force rebuilds on every incremental build. Real rules should
always depend on rules that have real outputs.

--warn_phony_looks_real will print a warning any time that a PHONY rule
has an output that looks like a file (has as / in it). This is can cause
confusion for users if it's not a real file, and it shouldn't be marked
as PHONY if it will be run every time (since that will slow down
incremental builds).

Test: run build_test on downstream branches, see few warnings
Change-Id: Id135c85c991976be33cd48a34eba0caaff1eda2c