ActivityManager: create process group in behalf of app zygote

app zygotes don't run under system pid/gid, so they can't create groups for
processes they spawn. Use system_server to create the correct process groups

Bug: 168907513
Bug: 62435375
Test: verified that webview processes are migrated to their own groups
Change-Id: Ifb912b42bca4d7670e6bb15510484296b772bb86