Add PRODUCT_BUILD_*_IMAGE, BUILDING_*_IMAGE to control building of images

These centralize the decisions on whether to build certain images or
not, and allow the product definition to override that choice.

There are a few use cases here:

 * For GSI-like cases, we only want to build the system image. This
   didn't really change, but it's somewhat simpler to configure, and
   easier to understand the build logic.

 * On the opposite side, when you're planning on using a GSI, the device
   specific build can only build the vendor images (or some other set).

 * Some cases (Fuchsia, etc) don't want to build any images, as they'll
   be distributing the build artifacts in their own packaging.

I suspect in the future, TARGET_BUILD_APPS may be able to be refactored
into the third use case.

Test: treehugger
Test: Create a product definition that includes nothing, try to build it.
Test: compare build-aosp_crosshatch.ninja and build-crosshatch.ninja before/after
Change-Id: I685ab841be3718d3dd7052c28ccd764bb6f1991a