Optimize build for PRODUCT_BUILD_SYSTEM_IMAGE := false case.

When PRODUCT_BUILD_SYSTEM_IMAGE is false, avoid building artifacts and
intermediates that will not be used. This speeds up the build for these cases.

Bug: 123427297
Test: No change when building system image, smaller build when not
Change-Id: I438e4794af5376c897ffcc1d795a1e114dccd351