DateTimeController: make possible functions const

MonthShortToString and DayOfWeekShortToString don't change the
underlying object. Those are just getters and can be declared `const`.

Author: NeroBurner

src/components/datetime/DateTimeController.cpp

@@ -108,11 +108,11 @@ void DateTime::UpdateTime(uint32_t systickCounter) {
   }
 }
 
-const char* DateTime::MonthShortToString() {
+const char* DateTime::MonthShortToString() const {
   return MonthsString[static_cast<uint8_t>(month)];
 }
 
-const char* DateTime::DayOfWeekShortToString() {
+const char* DateTime::DayOfWeekShortToString() const {
   return DaysStringShort[static_cast<uint8_t>(dayOfWeek)];
 }
 

src/components/datetime/DateTimeController.h

@@ -61,8 +61,8 @@ namespace Pinetime {
         return second;
       }
 
-      const char* MonthShortToString();
-      const char* DayOfWeekShortToString();
+      const char* MonthShortToString() const;
+      const char* DayOfWeekShortToString() const;
       static const char* MonthShortToStringLow(Months month);
 
       std::chrono::time_point<std::chrono::system_clock, std::chrono::nanoseconds> CurrentDateTime() const {