Android handler: allow to set the application context for the app being run.

Most of the Android applications (including the network service) will need to
access the application context in which they are run. This patch allows an app
to request the application context by exporting an appropriate
(InitApplicationContext) function, which will be called by the handler before
entering the MojoMain of the application.

BUG=436508
R=qsr@chromium.org

Review URL: https://codereview.chromium.org/763323002