Bind VR compositor service from VrManagerService.

- Allow VR system application to have VR manager service bind a
compositor process so that oom scoring for the compositor process can be
adjusted.

Bug: 38390640

Test: Check that the VrCore compositor process is -700 on AIO devices with:

  adb shell procrank -o

Change-Id: I7b5bce5eb87bd5ed0fdeadf4ec7bdd2b361ab875