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Move 4 CaptureSession test only functions to test file api. Use callback
instead of overriding ButtonListener, seems to be new style [1].

[1] https://bugs.chromium.org/p/chromium/issues/detail?id=772945

Test: existing tests
Bug: none
Change-Id: Ic61bd06419e1c9f348fa35d62e648d42ab1ec9e6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2480812
Commit-Queue: Sammie Quon <sammiequon@chromium.org>
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Cr-Commit-Position: refs/heads/master@{#818652}