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/readerui.lua

@@ -106,6 +106,8 @@ function ReaderUI:init()
     -- cap screen refresh on pan to 2 refreshes per second
     local pan_rate = Screen.low_pan_rate and 2.0 or 30.0
 
+    Device:setIgnoreInput(true) -- Avoid ANRs on Android with unprocessed events.
+
     self.postInitCallback = {}
     self.postReaderCallback = {}
     -- if we are not the top level dialog ourselves, it must be given in the table
@@ -457,6 +459,8 @@ function ReaderUI:init()
     end
     self.postReaderCallback = nil
 
+    Device:setIgnoreInput(false) -- Allow processing of events (on Android).
+
     -- print("Ordered registered gestures:")
     -- for _, tzone in ipairs(self._ordered_touch_zones) do
     --     print("  "..tzone.def.id)