Compute PPID at ninja time

Compute PPID at ninja time

Using $(shell $$PPID) to compute PPID will return the pid of kati, which
will be the same for multiple sequential runs of make without changing
any makefiles.  Replace it with a bash subshell command to return the pid
of ninja, which will be different for every call to make.

ART_HOST_TEST_DIR now contains $ that need to propagate to the final
bash command, and must be evaluated exactly once.  Escape the $ in
${ART_HOST_TEST_DIR} in define-test-art-run-test so that
${ART_HOST_TEST_DIR} is not substituted in the function call to prevent
evaluating its contents in the $(eval) that wraps it, which would
substitute out the necessary $.

Test: m -j test-art-host
Change-Id: I3303805481cc1c77d1a321674e8c7c003043a7c4