[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
Test: manual testing
Change-Id: I415a9c4a91da765db83a3755264b2b19679d0878