Add LauncherProvider interface to get and set current workspace layout via xml

=> For prototyping purposes only. The method is protected by system|signature permission.
=> Uses the AutoInstall layout xml specification to allow for getting and setting the current launcher layout.

Test: manual via prototype apk
Flag: EXEMPT prototyping interface

Change-Id: I04dd29ee69db642095dfb5f6c4965cdb8509b05e