[Android]: Update Makefile.android to use platform-14 as minimum.

The version of the NDK in Chromium only has platform/android-14 and above
(since Chrome only supports API 14 and above).  To make it easier for
developers to use the Chromium NDK to build V8, this CL updates the default
platform version to 14.

R=ulan@chromium.org

Review URL: https://codereview.chromium.org/267873002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21159 ce2b1a6d-e550-0410-aec6-3dcde31c8c00