SDK Manager: change initial selection logic.

SDK Manager: change initial selection logic.

This changes the way packages are selected:
- When using the "Select New" link, all new stuff is selected.
- When using the "Select Update" link, only installed stuff
  with updates is selected.
- When starts the SDK Manager, the heuristic becomes:
  - Select any updates we can find.
  - Check whether the top platform is installed.
  - If not at all, select all its packages. This should
    cover the "there's a new platform available" scenario
    by just selecting to install everything for it.
  - If the top platform has at least one item selected,
    make sure the platform package itself is installed or
    select it.
  - If the platform currently installed or a selected
    update lacks a system image, we want to suggest some
    kind of system image. For that, look whether the platform
    (current or selected update) provides one. If they don't
    select any system image package we can find.
  - On Windows, also suggest to install the USB driver.

SDK Bug: 20607

Change-Id: Ifab423b226ad708e9117eefd4d76033d866c57a8