SDK Manager: reuse complete downloads, retry Windows locks.

This resolves 2 main issues with the SDK updater:
- Completed downloads are not cleared till the install is successful
- They are also stored in SDK/temp rather than the real Windows TEMP
  folder, making them more discoverable for savvy users.
- There's a retry loop on failed install when due to a directory
  being locked.
- The retry loop comes with the a Big Fat Warning[tm] in a modal
  dialog box. You can't miss it. And it explicitly mentions the
  antivirus software can be the root cause.

SDK BUG 2235058

Change-Id: Id49751ebd67e7291a0e7005136b22576335729c1
3 files changed