Refactor multiproduct_kati

Move the actual per-product builds out of the main function, combining
the product configuration and build sections. This makes it a lot more
readable.

This ends up saving a lot of FDs -- a -only-config build on aosp with 77
products used to require ~500 FDs, it succeeds with a limit of 150 FDs
now. I'm leaving the code to bump our FD limit though, since 2x the
number of internal products is too close to the 1024 limit for comfort.

Bug: 70370883
Test: prlimit -n150:150 build/soong/build_test.bash -only-config
Change-Id: Ia559beadc19deb8a5b9d50af6e0276e846fd8608