adb: don't cache failure to get features.

When we're waiting for the device to show up (e.g. `adb logcat` with the
device not connected), we can transition from the feature set being
unavailable to being available.

Test: `adb logcat` with device disconnected
Change-Id: I5c93a725605c886cba2c66daa25b484c90a170ec
1 file changed