Prepare bolts.Task to be safe to be usde from Kotlin

Summary:
In this diff I analyzed bolts.Task class to ensure it's ready to be used from kotlin.
I won't convert bolts.Task to kotlin, but this is necessary to be able to convert its callsites (ReactHost) to kotlin

This is a 'continuation' of the previous diff, I splitted in 2 because Task is a very large class and it's worth analyzing separately.

bypass-github-export-checks

changelog: [internal] internal

Reviewed By: cortinico

Differential Revision: D46194126

fbshipit-source-id: 765af7683fa62814747c7d8516d311cffabac3c1