Gracefully handle unbind errors when detaching

When detaching from a previous wallpaper we unbind our connection from
the underlying service. This can generate an exception if the service is
no longer available and lead to the wallpaper state being left in a
half-cleared state as well as causing the detach operation to fail. This
CL adds a catch to handle the exception, in this case it doesn't matter
if the the service has gone away since we're detaching from it anyway.

Bug: 296508731
Test: atest WallpaperManagerTest
Change-Id: I19a3fedecafca10744cf21f2e6bd40c9f8f6350a