Move RegionSamplingHelper into WMShell shared

This is used by launcher for gesture nav & bubble bar handle. I wanna
use it for the handle on the bubble bar expanded view, which is in
WMShell. I don't think shell can depend on sysui shared lib since
that lib already depends on shell, so I'm moving this into shell.

Flag: EXEMPT moving a class from sysui to shell
Test: atest RegionSamplingHelperTest
Bug: 353160491
Change-Id: I994cb0f4103ebb9a88262069851aecfd2403d441