detect screen DPI for android devices

frontend/ui/device/screen.lua

@@ -1,3 +1,4 @@
+local Blitbuffer = require("ffi/blitbuffer")
 local Geom = require("ui/geometry")
 local DEBUG = require("dbg")
 
@@ -80,6 +81,10 @@ function Screen:getDPI()
         self.dpi = 265
     elseif model == "Kobo_pixie" then
         self.dpi = 200
+    elseif util.isAndroid() then
+        local android = require("android")
+        local ffi = require("ffi")
+        self.dpi = ffi.C.AConfiguration_getDensity(android.app.config)
     else
         self.dpi = 167
     end