Take into account RAM usage for multiproduct_kati

Apparently we have some instances of Soong taking 16GB of RAM. While
that's a problem to solve on it's own, make multiproduct_kati's
auto-parallelism detection smarter by taking into account the total RAM
on the machine instead of just the number of CPUs.

Bug: 146925549
Test: check soong.log for autodetected parallelism values
Change-Id: Ice34c2cf73ea4f8f235521cbefc8654a79a04eef