Binding to the RemoteViewsService directly from the host

The host passes an IServiceConnection to the AppWidgetManager which
is used to bind to the RemoteViewsService. This allows the host to
recieve the connection callbacks directly instead of proxying it via
the AppWidgetManager. The host is also responsible for unbinding to
the service.

Bug: 26481160
Test: adb shell \
  am instrument -w -e class android.widget.RemoteViewsAdapterTest \
  com.android.frameworks.coretests/android.support.test.runner.AndroidJUnitRunner

Change-Id: Iac400095a319c3a43714c82fda7516be1ccc68af