ReaderRolling: avoid ANR on Android when (re)rendering. (#8501)

Wrap document opening and re-renderings (which can block
the app for some time) with setIgnoreInput() to avoid ANR
on Android.
Any setting update that could possibly cause a re-rendering
should send the UpdatePos event, to ensure the re-rendering
happens in ReaderRolling:onUpdatePos() where precautions
are taken to avoid ANR.

frontend/apps/reader/modules/readerrolling.lua

@@ -867,16 +867,24 @@ function ReaderRolling:onUpdatePos()
         -- we have set above) to avoid multiple refreshes.
         return true
     end
+
+    UIManager:discardEvents(math.huge) -- Discard any past and upcoming input events for the next hour.
+    Device:setIgnoreInput(true) -- Avoid ANRs on Android with unprocessed events.
+
     -- Calling this now ensures the re-rendering is done by crengine
     -- so updatePos() has good info and can reposition
     -- the previous xpointer accurately:
     self.ui.document:getCurrentPos()
+
     -- Otherwise, _readMetadata() would do that, but the positioning
     -- would not work as expected, for some reason (it worked
     -- previously because of some bad setDirty() in ConfigDialog widgets
     -- that were triggering a full repaint of crengine (so, the needed
     -- rerendering) before updatePos() is called.
     self:updatePos()
+
+    Device:setIgnoreInput(false) -- Allow processing of events (on Android).
+    UIManager:discardEvents(true) -- Discard events, which might have occured (double tap).
 end
 
 function ReaderRolling:updatePos()