Don't destroy usb debugging dialog for emulator if usb is not connected.

ACTION_USB_STATE is always false for emulator, which causes the dialog
to appear and then immediately disappear.

Bug: 187862091, 160861855

Test: adb kill-server; rm <adb keys>; adb devices (should popup dialog)
Change-Id: Ia21533d8b8b5d10526f1789abdffe4bbe32d7025