devlib/AndroidTarget: Move adb specific commands into the ADB connection

The `AndroidTarget` class should not depend on ADB specific commands as
is is possible to use this target with other connection types e.g. ssh.
Therefore move the adb specific commands into the `AdbConnection`.

- `wait_for_device` and `reboot_bootloader` are now exposed in AndroidTarget
as generic methods and call through to the connection method.
- `adb_kill_server` is now a standalone function of the AdbConnection.

doc/target.rst

@@ -647,6 +647,14 @@ Android Target
    Returns ``True`` if the targets screen is currently on and ``False``
    otherwise.
 
+.. method:: AndroidTarget.wait_for_target(timeout=30)
+
+    Returns when the devices becomes available withing the given timeout
+    otherwise returns a ``TimeoutError``.
+
+.. method:: AndroidTarget.reboot_bootloader(timeout=30)
+    Attempts to reboot the target into it's bootloader.
+
 .. method:: AndroidTarget.homescreen()
 
    Returns the device to its home screen.