Clean up surfaces when app is resumed without being stopped.

When quickly toggling between two apps, app could be resumed while
it's stopping but not yet stopped. Upon resuming, it could have
surfaces that's marked mDestroying and waiting for the stopped
to be destroyed.

We need to dispose these surfaces properly. If the window is already
removed, we destroy them. Otherwise, clear mDestroying flag so that
the window is ready to be used again. Leaving mDestroying=true makes
the window ineligible for certain things such as receiving wallpaper.

bug: 30255354
Change-Id: Id881653550595ab8e702d6950949bf202ac5a0d9