adb: fix feature sets on devices that don't have them.

Previously, we weren't distinguishing between nonexistent devices and
devices that don't support features, resulting in lots of stuff being
broken for pre-N devices.

Bug: http://b/154272158
Test: adb shell on a JB emulator
Change-Id: I0058b2a60678e1ad0503f5abac30157be976c9c4