Removes BackgroundShellApplicationLoader from application_manager tests

BackgroundShellApplicationLoader is android specific. I'm removing use
from application_manager_unittests so that I can move
BackgroundShellApplicationLoader (see
https://codereview.chromium.org/788243007/ ). There is only one test
in application_manager_unittests that relied on
BackgroundShellApplicationLoader: BDeleted.

BDeleted needed BackgroundShellApplicationLoader because resetting the
loader (BackgroundShellApplicationLoader) triggered shutting down
the background thread and background message loop (owned by
BackgroundShellApplicationLoader). Shutting down the MessageLoop
closes any pipes. As A/B are InterfaceImpls they are destroyed when
the pipe closes. Removing BackgroundShellApplicationLoader breaks
BDeleted because resetting the loader no longer triggers closing
pipes. The fix is to make Tester own the TestAImpls it creates. This
way when the test resets the loader the TestAImpls are destroyed so
that the other side (TestBimpl) sees the connection error and cleans
up.

BUG=none
TEST=none
R=qsr@chromium.org, davemoore@chromium.org

Review URL: https://codereview.chromium.org/812083002