android: remove creation of background screenshots

Change-Id: I23a2bc875229546c72708eefca68a12099c2a7af
android: add to DisplayPortMetrics position and zoom getter

Change-Id: I90674f51113c082d3bd88a72f8b9883b44569f87
android: calculate tile position/dimension for FixedZoomTileLayer

Change-Id: I54664a9289ef513fb4660b002f6ce652c7a9e6ef
android: skip tiles that are already outside of viewport

Change-Id: I65f286797b459f30885560577a4dcb3d1108c0bf
android: clear tiles when adding a new tile, various

Change-Id: Ifc6e01a1beb39187a5231019d1bf7ac816052e78