vpx_ios/VPXExample: Disambugate ownership and usage of VideoBuffer struct.

Make VideoBuffer a member of VideoBufferPool.

Change-Id: I15d61633146287f03077f9d75c97ec6fe446d2c8
vpx_ios/VPXExample: Use tap instead of touch in instructions.

Per Apple UI guidelines.

Change-Id: Ied81153c4c3d9edb8237b9b892c28e98215e267a
Merge "vpx_ios/VPXExample: Use tap instead of touch in instructions."
vpx_ios/VPXExample: Fix some nits.

- Fix include order (one place).
- Remove unnecessary include (one place).
- Remove extra new lines (two places).

Change-Id: Id4a8841eb77d65d59c69fa3103742ca39909e41c