Remove Android system ICU support.

We don't support using the system ICU on android any more (the library
is not included in the NDK and so cannot be relied on). Remove the
support for it in ICU gyp files, to eliminate remaining references to
the deprecated "android_src" gyp variable.

BUG=
R=mkosiba@chromium.org

Review URL: https://codereview.chromium.org/690603002

git-svn-id: https://src.chromium.org/chrome/trunk/deps/third_party/icu52@292738 4ff67af0-8c30-449e-8e8b-ad334ec8d88c