Provide a prebuilt binary of webview, replacing the version built from
source in frameworks/webview/chromium/Android.mk. The cleanspec will
delete any old libwebviewchromium.so installed systemwide and any
symlinks to it that may have been in /system/app/webview/lib, as the
prebuilt's library is directly stored in the webview lib directory
instead of in /system/lib.
The initial prebuilts here are from ub-webview-m40 build 1628204 and are
based on Chromium 40.0.2214.27.