startTethering with silent entitlement in systemui

During R development cycle, default entitlement check is changed from
silent to showUI(start activity) in TetheringRequest. So start hotspot
by systemui is changed from silent to trigger UI. Expcility set
entitlement back to silent check in systemui for better user experience.

Fix: 182724427
Fix: 169493230
Test: atest HotspotControllerImplTest
      atest CtsTetheringTest
      on/off hotspot by systemui
Change-Id: Ida1964e28a7b2d0f1a1394e5de2ec437e0bc420a