Test for dangling rules in make checkbuild

Get a list of leaf nodes in the dependency graph
from ninja, and make sure none of them are in the
output directory.  This ensures that there are no
rules that depend on a file in the output directory
that doesn't have rule to generate it.  The check
will catch a common set of build failures where
a rule to generate a file is deleted (either by
deleting a module in an Android.mk file, or by
modifying the build system incorrectly).  These
failures are often not caught by a local incremental
build because the previously built files are still
present in the output directory.

Bug: 36843214
Bug: 68062417
Test: manual
Change-Id: I4933187e8b72f2ef0c32d18ffea756e2c6fa417c