Add a command line interface to SystemUI

This CL adds some basic support for a command line interface in
SystemUI, using `adb shell cmd` as the jump-off point.

The basic design is kind of as follows:

[ shell ]  -> [ system_server ] -> [ StatusBarService ] -> [ SystemUI ]
                                                                  |
             [ shell output ] <- [ sbm service ] <- [ CommandRegistry ]

Where StatusBarManagerService implements a new `passthrough` interface and
forwards all remaining arguments to SystemUI. SystemUI then spawns a new
thread and sends the commands off to the CommandRegistry. Commands are
parsed and executed from there, and all output is written to the given
ParcelFileDescriptor, which is piped back to SBMService using a
TransferPipe.

Commands will by default run on the main executor, but can register with
an executor in order to send work off to any other thread. The command
registry thread will block on the completion of the command.

Implementing new commands as of now is trivial: create a new concrete
implementation of CommandLine and add it to the command map inside of
CommandRegistry.

Test: `adb shell cmd statusbar prefs list-prefs` should have output

Change-Id: I189fa55df6caabe25d693c2241a4d994e7473a2c