Preserve aar files for support library and extras

The build system can consume .aar files, and with aapt2, does not need
extracted resources, but does need dependency information. We still need
the extracted resources for aapt(1) users, until those can be converted.

The pom2mk tool handles writing out Android.mk files based on maven
repositories, including dependency information.

./update_current.py 4023242 -c
./update_current.py 4295864 -s

Bug: 64723465
Test: m ANDROID_BUILDSPEC=vendor/google/build/app_build_spec.mk
Test: mmma prebuilts/sdk/current ANDROID_BUILDSPEC=vendor/google/build/app_build_spec.mk
Change-Id: I408eca67ecb3728ed067cd0f7309fc8cdb5f043f
diff --git a/current/extras/Android.mk b/current/extras/Android.mk
new file mode 100644
index 0000000..5053e7d
--- /dev/null
+++ b/current/extras/Android.mk
@@ -0,0 +1 @@
+include $(call all-subdir-makefiles)