Make Android multiprocess tests use IChildProcessService.aidl

Removing the multiprocess test specific ITestClient.aidl and making the
multiprocess launcher and service classes use IChildProcessService.aidl.
This is in preparation of the move of child process launcher to base and
having the multiprocess tests use it.

BUG=702316

Change-Id: I5cb81c45dcf1d4f19c34fbaa42194b22ca358a5f
Reviewed-on: https://chromium-review.googlesource.com/534953
Commit-Queue: Jay Civelli <jcivelli@google.com>
Reviewed-by: Tommy Nyquist <nyquist@chromium.org>
Cr-Commit-Position: refs/heads/master@{#479796}
4 files changed