FileDownloader

feat(task): add 'isUsing' and 'isRunning' for a task

chore(upgrade): upgrade version name(0.2.7->0.2.8) code(18->19)

chore(gradle): upgrade gralde tool version 1.5.0->2.1.0

chore: add ParcelCreator to lintOptions, it don't need parcel creator for the sub-class of MessageSnapshot

refactor(internal-communication): reduce alloc-new-object, take a message snapshot for downloading statuses updating through whole communication architecture