Add an incremental mode to multiproduct_kati / build_test

This uses a very large amount of intermediates, but can speed up
subsequent multiproduct_kati runs by not re-reading all the makefiles
unless necessary.

Since we'll no longer have the full build output on every run,
explicitly save away a std_full.log of the log from the last time we ran
Kati.

Test: build/soong/build_test.bash -products aosp_arm -incremental
Change-Id: Iaae580c0e49a2024528617d39b2b477e29d99250