Add target pinning to widget

[yajasmalhotra]

Overview:

This change is intended to improve the Smartspacer experience on more devices. Since not all OEMs support Native Smartspace (e.g. Samsung, Xiaomi, Huawei, users on these devices can not scroll through targets, instead having to enable button-based navigation.

Implementation

All that said, some OEMs do support stackable widgets. This change lets users pin individual targets to widget instances, letting them stack multiple instances and scroll through those. There is the obvious downside that users will lose the seamless experience of having targets take priority by themselves. For those who want to let Smartspacer use its existing logic for target sorting, you can also just pin widget instances to page numbers, letting the app dynamically prioritize targets to display.

[yajasmalhotra]

@KieronQuinn feel free to req changes