Make unbundled support library builds more unbundled.

Allow AOSP builds of support library to use remote
maven repositories, additionally, expect users to have
their own copy of the SDK.

This is all dependent on having frameworks/support/unbundled-build
which developers can copy from development/unbundled-build.
I'll add this to README.md once everything is up and running.

This is a cherry-pick of r.android.com/593379

Test: open project in studio and it works \o/
Change-Id: I479522942ce3aa3c3137dbf6aacdabe08bb733a8
diff --git a/.gitignore b/.gitignore
index 7e44717..ac3565c 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,3 +1,4 @@
+/unbundled-build
 .classpath
 .gradle
 **/.idea/**