Add support for device emulation in the Gallery app

This CL adds support for device emulation in the Gallery app so that we
can easily emulate any device on another device, e.g. emulate a phone on
a tablet or a emulate tablet on a phone.

The emulation is done at the device level, so that might even be useful
outside of the Gallery app, e.g. when working on SystemUI.

See b/231131244#comment11 for a quick video.

Bug: 231131244
Test: Manual
Change-Id: I64094369c32b796ad41a274d23e0910b7c61a3b0