Handle finding zipalign more gracefully
Handle finding zipalign more gracefully

Summary:I don't know how standard ANDROID_SDK is, but I've been able to build a
lot of open-source apps without setting it.  Let's try PATH as a fall back, and
if we still don't find zipalign, just warn, don't fail completely.

Reviewed By: newobj

Differential Revision: D3105817

fb-gh-sync-id: 5cca07ab8d2b1e8d6ba71d407e3b580944c13224
fbshipit-source-id: 5cca07ab8d2b1e8d6ba71d407e3b580944c13224

facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex