Finish Ubuntu Touch developer mode including UI support

UDS discussion notes:

 Needed for developer mode:
 - UI Options -- in settings?
 * shell access (adb + SSH) (RNDIS and/or Wifi?)

* make adb run as phablet user (there should be a bug open about this) so that the user can use sudo
* fingerprint verification for adb connections

* USB gadget is setup in Android container, system settings would need some way to access that
  - could have a wrapper script that we can sudo?
  - or could design a clean dbus service to expose android properties

* Do we want to move the gadget handling code outside of android? not a big benefit as it's device specific code

Platform developer issues we might want to solve:
* MTP service resets the ADB connection -- SOLVED by Sergio already
[Sergio, Oliver] * starting adb early on for platform porting by putting some file in place that gets recognized during boot
[Oliver, Stephane] * having some way to enter the android container; lxc-console should work but doesn't

We need a way for QA to enable adb by default before the first boot as to allow them to run their tests

Would we nice not to have to specify -d when using phablet-flash against recovery

SSH should be startable via property system.

Work items:
[sergiusens] starting adb early on for platform porting by putting some file in place that gets recognized during boot: TODO
[rsalveti] make lxc-console work for the android container: DONE
[ogra] DBus service to expose specific Android properties to the end-user: DONE
[ogra] Add a setting panel to enable/disable adb over DBus to the new service: DONE
[sergiusens] add an ubuntu_command to enable adb: DONE
[sergiusens] add a copy of the android properties database to the recovery so that getprop works on ro.model: TODO
[ogra] make adb run as phablet user and only when a user password is available: DONE
[ogra] provide a user accessible way to toggle sshd on/off without requiring sudo: DONE
[ogra] add UI to popup the device fingerprint when negociating a connection: DONE