Optimize mutator contexts

Mutator contexts are created for every module, and in the case of
transition mutator contexts for every dependency of every module.
Unlike the Blueprint mutator contexts that they wrap, the Soong
mutator contexts can be relatively large.  Add global pools for all
of them that can avoid the repeated allocations.

Test: SOONG_PROFILE_MEM=/tmp/mem.pprof m nothing
Change-Id: I64a5f3c91292cff6352300f99c11ac50c713f96d