camera2-portability: Optimize out some camera2 AE precaptures

If it's already converged and the flash isn't being forced on, there's no need
to force the precapture routine, which would delay our capture significantly.
Also use onCaptureProgressed() instead of onCaptureCompleted() when possible.

Change-Id: I0bb7956498b0070382cd5b39b7ccdc0cc0765913