flo: Always set AE_MODE_ON_AUTO_FLASH as scene mode override

All scene modes currently need AE_MODE_ON_AUTO_FLASH. Revisit this
on a per-scene mode granularity if a scene mode is defined in the
future that needs a different setting.

Bug: 10951740
Change-Id: I8d43a54a0f217d6e7e9be1dca568033cac9ba93f