Changed the way the native main function is invoked so that the main stub
is not required anymore. Launching in the sub-process is now done from Java
by calling into the new MainRunner class whose JNI code is in the same
module as the one used to start the test natively and is already linked
with main().

BUG=666356

Original issue's description:
> Multiprocess test client: Android child process launcher rework.
>
> Changing the way test start child processese on Android.
> Forking natively should not be used on Andoid, instead child processes
> should be started through the Android framework, which requires going
> to Java and using Android services.
>
> For that we introduce the MultiprocessTestClientLauncher java class
> which starts a service in a new process. We provide several
> MultiprocessTestClientService service classes to ensure we can have
> multiple child processes.
> File descriptor are passed from the parent to the child process as
> Parcelable, and we use the GlobalDescriptors class to register them.
>
> TEST=Run the mojo_system_unittests
> BUG=666356
> Review-Url: https://codereview.chromium.org/2549363004
> Cr-Commit-Position: refs/heads/master@{#441799}
> Committed: https://chromium.googlesource.com/chromium/src/+/5513e4622febbc838dfebddb51347ab797747836

Review-Url: https://codereview.chromium.org/2611323002
Cr-Commit-Position: refs/heads/master@{#442477}