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This addresses a subtle race condition in
CameraModuleCallbacks::SetCallbacks().

The key sequence of events,
1. CameraHalClient receives a pending remote of CameraModule
2. CameraHalClient calls CameraModule::GetNumberOfCameras()
3. CameraHalClient calls CameraModule::SetCallbacks() to pass a pending
   remote of CameraModuleCallbacks.
4. CameraModule receiver would fire
   CameraModuleCallbacks::CameraDeviceStatusChange()
5. CameraModule receiver then fires CameraModule::SetCallbacksCallback

The expectation was that 4 would be received before 5. However, since
CameraModule and CameraModuleCallbacks run on different message pipes, 4
can actually be received after 5, not meeting the expectation.

So here we make CameraModuleCallbacks an associated interface of
CameraModule.
Please see crrev.com/c/2434712 for the implementation on CameraModule.

Bug: b/169324225
Test: With the following,
    1. Open Chrome Camera App (CCA)
    2. restart cros-camera
    3. Verify CameraDeviceStatusChange is received before
       SetCallbacksCallback
    4. Repeat and passes 20/20 times.
    Additionally,
    1. Build capture_unittests and scp to DUT
    2. tast run dut 'camera.CaptureUnittests'

Change-Id: I79f6006b7827d2e556b01e7d57df43916ca7913e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2470898Reviewed-by: Wei Lee <wtlee@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#841019}