SDK Manager: correctly handle lack of AvdManager

SDK Manager: correctly handle lack of AvdManager

Digit's issue in SDK Bug 2535112 was that AndroidLocation failed
to get the SDK home dir; consequently the SdkManager failed to
create an AvdManager and was using a null ptr everywhere.

This CL addresses the core issue here:
- The New AVD button should be grayed if there is AvdManager.

- UpdaterData.initSdk was catching the error and printing an
  error log that is not shown in the UI. There's now an explicit
  dialog box that explains the error and how to solve it.

- UpdaterWindow now aborts if the above dialog is shown. There's
  no need to keep the SDK Manager window around -- OK maybe that's
  a bit harsh since the SDK download/install part can stil lbe used
  by do we really want to deal with an half-functional window?
  No we don't really want that use-case. Let's force the user to
  fix it.

- Fixed the AndroidLocation error message to something more usable.
  Most people can set an environment variable, however few know
  how to "set up the user.home property" (I don't even know how
  to do that.)

- Display the AVD base folder in the AVD page.

Change-Id: I2b9dbc7180e4bb3ff696de257bb260e808301d2d