A few minor fixes after #4847 (#4850)

* Add a toggle to disable the C blitter in the Dev menu (depends on https://github.com/koreader/koreader-base/pull/882) (never shown if the JIT is disabled, grayed out if the C blitter is not installed)
* Fix a few sizeUtf8Text call sites that were doing a nil check in order to account for the new return type.
* Tweak statusbar handling to avoid spurious sizeUtf8Text warnings when it's hidden, and unify its behavior between being hidden via toggle, and hidden on book open (at least when all-at-once is not enabled).
* c.f., https://github.com/koreader/koreader-base/pull/882 (Android, PB, RGB32 & Legacy Kindle regression fixes).

platform/android/llapp_main.lua

@@ -3,11 +3,8 @@ A.dl.library_path = A.dl.library_path .. ":" .. A.dir .. "/libs"
 A.log_name = 'KOReader'
 
 local ffi = require("ffi")
+local dummy = require("ffi/posix_h")
 local C = ffi.C
-ffi.cdef[[
-    char *getenv(const char *name);
-    int putenv(const char *envvar);
-]]
 
 -- check uri of the intent that starts this application
 local file = A.getIntent()
@@ -26,7 +23,7 @@ A.execute("chmod", "755", "./tar")
 A.execute("chmod", "755", "./zsync")
 
 -- set TESSDATA_PREFIX env var
-C.putenv("TESSDATA_PREFIX=/sdcard/koreader/data")
+C.setenv("TESSDATA_PREFIX", "/sdcard/koreader/data", 1)
 
 -- create fake command-line arguments
 arg = {"-d", file or "/sdcard"}