Wait for children to exit in makeparallel

This is a partial revert to the last makeparallel change, since now we
were being killed by the signal before waiting for our child to exit. So
instead of not installing the handlers, only pass the signal along if
it's a SIGTERM.

Bug: 35214134
Test: Ensure that we're still only getting one signal for SIGINT
Test: Ctrl-C, ensure that all the soong_ui lines are before the make
      error line.
Change-Id: I26fff9483a3abfd79ceb5a9ea47e3f7572d9e923