Create a highmem pool and put metalava into it

Create a highmem pool based on the total RAM and the number of CPUs,
with an override via the NINJA_HIGHMEM_NUM_JOBS variable.  Put
metalava into the highmem pool.

Ninja does not support nested pools, and when goma or RBE is enabled
the maximum ninja parallelism is set very high with local jobs in a
local pool.  When both the local pool and highmem pool are enabled,
the total number of local jobs will be as high as the sum of the sizes
of the two pools.  Keep the highmem pool limited to 1/16th of the
local pool when remote builds are enabled to try to minimize the
effect while still limiting highmem jobs.

Fixes: 142644983
Test: m nothing, examine pools
Test: m USE_GOMA=true nothing, examine pools
Change-Id: Id79f11f44948992960ac34ecf831dacbe21bd332