When BinderProxy is passed in as the IBinder for getEnclosingActivityContainer the activity manager cannot turn it into an ActivityRecord. This causes NPE in ActivityThread which is Not Good (tm). Allowing null to be returned when requesting an ActivityContainer and handling it appropriately fixes this bug. Fixes bug 12473669. Change-Id: I6937636042f8853b3ddc2df40be010e7391e41a5