Refactor Kati logic

Make the cleanspec & Android.mk runs of kati use much of the same code
and arguments.

Also renames 'Kati' to 'KatiBuild' in many cases, in the the expectation
that we'll have a 'KatiPackage' step in the future.

Use --no_ninja_prelude and move local_pool & _kati_always_build_ into
the combined ninja file. This will reduce the need to re-read makefiles
when Goma is enabled, and it allows us to include more than one
Kati-generated ninja file in the build graph.

Bug: 116968624
Test: build_test on downstream branches
Change-Id: Ibdac689b81f62dc293647fad917d84946f2c3cfa