Attribute planeAlpha was added with 1.2 but later

The addition of the planeAlpha attribute is one of the bad stories about the HAL
compatbility between different Android versions. It was added after 1.2 with jelly bean
but not marked as part of 1.2. It doesn't break the ABI. We're adding support for it
now depending on the Android version (>= 4.3).

Signed-off-by: Simon Busch <morphis@gravedo.de>