Android handler: inject the application loading part from the shell .so.

To properly set thunks in the application being loaded, we need make_thunks() to
be called from an .so that has them set itself.

This patch extracts the application loading logic from the bootstrapping library
to the Mojo Shell .so. When an application is being bootstrapped, we inject the
loading function by passing a function pointer.

BUG=436508
R=qsr@chromium.org

Review URL: https://codereview.chromium.org/772393003