Add support for incremental annotation processing with Gradle 4.7

[simtel12]

Same as rharter/auto-value-parcel#131: As Gradle 4.7 is out, there had to be an issue for this :)
The documentation to handle this is here.

I think auto-value-gson could implement this? (but I might be wrong as I don't fully understand all the works behind annotation processors).

[3flex]

May be worth waiting to see what happens in auto value first... google/auto#615

[bingranl]

Auto Value supports incremental annotation processing in 1.6.3-rc1. Extension can implement new API to support incremental annotation processing. See #673