Disables Back gesture while Sandbox is running.

We detect this gesture within the Sandbox itself,
to properly educate the user when they do it
correctly or make a mistake.

Test: Launched GestureSandboxActivity and verified its
gesture handling/animations were used and the standard
onBackPressed callback was not called. Verified normal
behavior did still work in other apps.

Bug: 148542211
Change-Id: Ia8be5229ad6db00b84c8fdc082ae861cb4c05109