Replace explicit intent with implicit ACTION_TETHER_SETTINGS intent

Using implicit intent will give us the freedom to handle it differently
on Settings app side at runtime. This won't change the behavior of this
QuickSettings Hotspot Tile.

Bug: 149590956
Test: Built and flashed to crosshatch. Long clicking on Hotspot Tile
will go to Tether Settings.

Change-Id: I9f79ff7966b65f9a2ab3d69dfb4a5f50cad798d4