Have test-art-host-dexdump specifically depend on the host binaries.

dexdump2 and dexlist2 are phony targets representing both the host and
target binaries.
Also set environmental variable ANDROID_HOST_OUT=$(realpath $(HOST_OUT))
before calling art/test/dexdump/run-all-tests.
art/test/dexdump/run-all-tests references ANDROID_HOST_OUT, which may be
not set up by the user if he didn't run build/envsetup.sh.

Bug: 22343844
Change-Id: I38b5f3efccd5817d7d9ff0e91b5c6d070b546d77
diff --git a/Android.mk b/Android.mk
index ff39d53..c01464a 100644
--- a/Android.mk
+++ b/Android.mk
@@ -240,8 +240,8 @@
 
 # Dexdump/list regression test.
 .PHONY: test-art-host-dexdump
-test-art-host-dexdump: dexdump2 dexlist2
-	art/test/dexdump/run-all-tests
+test-art-host-dexdump: $(addprefix $(HOST_OUT_EXECUTABLES)/, dexdump2 dexlist2)
+	ANDROID_HOST_OUT=$(realpath $(HOST_OUT)) art/test/dexdump/run-all-tests
 
 # Valgrind. Currently only 32b gtests.
 .PHONY: valgrind-test-art-host