Add the Share App system shortcut to the Go Launcher

This shortcut allows users to initiate peer-to-peer app sharing from an app's long-press menu on the home screen. It does so by passing an intent to the specified component. In this base implementation, that specification in config.xml is left blank, and the button will not appear. In GMS Launchers such as LauncherGoGoogle, the config should be set to point to Nearby Sharing.

This also establishes Launcher3QuickStepGo as a distinct subclass of QuickstepLauncher instead of merely a build configuration.

Bug: 157515296
Bug: 174286192
Test: Manual (tested on local wembley devices)
Test: m -j RunLauncherGoGoogleRoboTests
Change-Id: I826bd03721d2e7e963b15d5958e9a18a1cc16f1d