Camera: Prevent double-stops on preview for some HALs

The OMAP3 HAL (at least) stops the preview while taking a snapshot,
without generating events to the upper layers, and telling it to
stop again leads to an inconsistent state.
Use "previewStopsDuringSnapshot" to avoid sending a stop while
it is already stopped

Change-Id: I4d4fe501c0dd6dd9aefb7d35f0e9dc6887e15162