AVD Manager: no longer provide addon's base target sys-img.

When an add-on did not provide any system image, the AVD
Manager behavior was to automatically provide the parent
platform's system image instead. The new behavior is to
not provide any system image and consequently the user
cannot create an AVD using such an add-on.

Change-Id: Id5e27c3e7c6d57bccba6cb318e1f318f4b5c356e
(cherry picked from commit 222fa2d5b0f59baef96360affabc84656accb7c6)
1 file changed