Switch genrule-phony to ninja phony rule

Instead of using android.Phony, which is a "real" rule, use
blueprint.Phony, which is ninja's built-in "phony" rule. That's desired
in this case, since we only want this to be an alias to the inputs.

Test: cd system/bt; mma; mma (second run had nothing to do)
Test: treehugger
Change-Id: Ib0b630e23653c05a12cf617350f8b4c6d6d6cdff