Pass --writable to kati

These warnings are usually hit in one of the two conditions:

1. Targets that should be marked .PHONY, but are not.
2. Attempts to define output files in the source tree.

See https://android.googlesource.com/platform/build/+/master/Changes.md#phony_targets
for more information.

Test: multiproduct_kati in all downstream branches
Change-Id: Ibe37cce320612825c22987e94d0740e8c0565b6f