Revert d50021d045df809aa8e6debf1a7e5727dabcd95c

It is safe to use prealloc for default, because of legacy fs (e.g.,
vfat) usage in Android.

src/OptionHandlerFactory.cc

@@ -376,12 +376,7 @@ std::vector<OptionHandler*> OptionHandlerFactory::createOptionHandlers()
     OptionHandler* op(new ParameterOptionHandler
                       (PREF_FILE_ALLOCATION,
                        TEXT_FILE_ALLOCATION,
-#if defined(__ANDROID__) || defined(ANDROID)
-                       // V_PREALLOC is too slow for Android device
-                       V_TRUNC,
-#else // !(defined(__ANDROID__) || defined(ANDROID))
                        V_PREALLOC,
-#endif // !(defined(__ANDROID__) || defined(ANDROID))
                        std::vector<std::string>
                        (vbegin(params), vend(params)),
                        'a'));