ActivityManagerService: Rather than throw an exception, return

INVALID_STACK_ID from getWorkspaceId when there is no associated
workspace. This leaves the caller free to default.

PhoneWindow: Default to FULLSCREEN_WORKSPACE_STACK_ID in the case
of INVALID_STACK_ID returned from the ActivityManagerService.

Bug: #23171731

Change-Id: Id5132337459bd2ef81b2416434d7657b6f3d1855