Turn on ninja phony outputs

This replaces the _kati_always_build_ hack with a ninja feature so that
ninja can actually understand what's happening. This means that we can
turn on some more options and checks around expected output filenames:

* Remove the output file(s) before the command executes
* Error if the output file(s) don't exist after the execution
* Error if the output is a directory

They're turned on immediately during the soong bootstrap runs, as those
run a limited number of rules. The main ninja execution does not remove
the output files, and prints warnings instead of errors for the others.
I'll turn them on as we understand how often those warnings are seen.

Test: m (check for new warnings)
Test: treehugger (check for new warnings)
Change-Id: I7f5c1a361dd1498eb54a2c07a918f3b0aa086e4c