Allow n2 as a replacement for ninja in builds

If `SOONG_USE_N2=true` is set in the environment, then n2 is used in
place of ninja.  Some ninja features are not available in n2 at this
time, but this enables bringup efforts to happen in parallel.

Bug: 352368206
Test: manual
Change-Id: I8455cb24eb640a4651782ee76e48a7d3a9932b93