Revert "Compute PPID at ninja time"

This reverts commit b1c17bb080ccf41b30909c0401df59e6f3a1ebb1.

It breaks make test-art-host when /bin/sh is not bash, as 
/bin/sh -c /bin/bash -c echo $PPID 
returns the pid of /bin/sh, not the pid of ninja.

Test: none
Change-Id: Ia4e5f76ea4c5f5b5c102ce54ae03c3fccb23a49f
diff --git a/build/Android.common_test.mk b/build/Android.common_test.mk
index 06485ca..449502c 100644
--- a/build/Android.common_test.mk
+++ b/build/Android.common_test.mk
@@ -21,9 +21,9 @@
 
 # Directory used for temporary test files on the host.
 ifneq ($(TMPDIR),)
-ART_HOST_TEST_DIR := $(TMPDIR)/test-art-$$(echo $$PPID)
+ART_HOST_TEST_DIR := $(TMPDIR)/test-art-$(shell echo $$PPID)
 else
-ART_HOST_TEST_DIR := /tmp/$(USER)/test-art-$$(echo $$PPID)
+ART_HOST_TEST_DIR := /tmp/$(USER)/test-art-$(shell echo $$PPID)
 endif
 
 # List of known broken tests that we won't attempt to execute. The test name must be the full