Disable task snapshot when image size is invalid

Since devices such as IoT can have no screen, the size of
graphic buffer can be 1x1. In this case the WindowManager crashes
while making bitmap with reduced size.
To prevent the crash, disabled snapshotting on this screen.

Bug: 62401135
Test: Device without the display does not crash when app dies.
Change-Id: I367008405e6238baf3e56974f7d363ea81dfa916