EVToolbox: Add an option to track updates.

 * Includes: Releases, nightlies, and test builds for each device.
 * Allows for multiple simultaneous downloads.
 * Downloads are tracked via the UI using the Android DownloadManager, which
   also supplies a notification for progress (to be removed in the future).
 * All downloads are housed in SDCARD:EVupdates/{gapps, nightly, release,
   testing}/
 * TWRP is exclusively supported for now, other support may follow.
 * Scheduling to check for updates is working / implemented. But
   notifications will be added in the future.

Change-Id: I05e429b898cb376aa53490bb9ab41e58010c962a