Gradle project file.

Adds a gradle project file so that other projects can use the
volly library.

Change-Id: I22d68fff50f9073474eb6be692bf7cbd08a366ea
Merge branch 'master' of https://android.googlesource.com/platform/frameworks/volley
If getting a 304, make sure you merge cached response headers with 304 headers

This is to prevent losing Content-Type