Restrict genrules to disallow directories as input

While Bazel genrules will allow genrules to accept a directory as input,
the results can be unexpected to a user as changes to the contents of
the directory may not trigger a rebuild as expected. Restricting this
in Soong ensures that converted targets will behave as expected.

Test: CI
Change-Id: I8616f58d1df267005e6c0ca3f4730d06de52c0d9