Add a GSI/DSU Loader to the developer option.

The Generic System Image (GSI) is a pure Android implementation with unmodified
Android Open Source Project (AOSP) code. The Dynamic System Updates (DSU) is a
Android Q feature that can download a system image and boot into it without the
factory rom being corrupted. This CL adds a DSU Loader which is a friendly
front-end that offers developers the ability to boot into GSI with one-click.
The DSU loader also offers the flexibility to overwrite the default setting
and load OEMs owned images.

Bug: 140090894
Test: click settings->developer option->debug::DSU Loader
    make -j32 RunSettingsRoboTests
Change-Id: Ia2a1b69b52047dd841dedf7f07b07f7ad3e65d46