android: Build against NDK prebuilts, not system libraries.

To allow the chromium WebView to build unbundled, we need to stop
linking to system libraries and instead link to the prebuilt libraries
in the NDK.

All modules compiled for target now specify LOCAL_SDK_VERSION with a
valid version of the SDK (defaulting to 19, the oldest version on which
the WebView builds). This can be overridden with the generator flag
"aosp_sdk_version".

Instead of converting library flags of the form "-lfoo" to entries in
LOCAL_SHARED_LIBRARIES, which causes a dependency on the system version
of the library, simply pass these flags through in ldflags. The NDK
library path is set automatically if LOCAL_SDK_VERSION is defined and
the correct version of these libraries will be found.

BUG=chromium:409851
R=primiano@chromium.org, thakis@chromium.org

Review URL: https://codereview.chromium.org/571053003

git-svn-id: http://gyp.googlecode.com/svn/trunk@1990 78cadc50-ecff-11dd-a971-7dbc132099af