[Magnifier-77] Show it on long press selection

Before this CL, the TextView magnifier was shown when selecting text,
but only when a selection handle was explicitly grabbed. This CL also enables
the magnifier when selection is performed by dragging after long press.

Bug: 119819194
Change-Id: I9546023723ef9ff84dbc6ca4c8fe5642c66a0146
Test: manual testing