Camera: Avoid Metadata re-allocation during shrinkCaptureResult

If capture result metadata is to be shrinked, reserve enough space so
that CameraMetadata isn't re-allocated when std::vector expands.

If vector expansion happens, CameraMetadata is re-allocated, causing
camera_metadata_t being re-allocated as well.

Test: Camera CTS
Bug: 109672123
Change-Id: Idaf5bf409bdbbccf617614edc96c0478260b4c30