handle multiple timer states

- feat: separate timer actions from states by adding a new class TimerEvent to hold actions Reset, Start, Pause, Finish, Refesh that could be performed on a timer.
- feat: handle multiple countdown timers in the App file by creating a map of the timer id to the countdown timer.
- fix: use gson instance from  TypeAdapter in Room's Converters class
- ref: remove scroll view parent from each timer item, a fix for the keyboard obscuring a label will be implemented in a future commit

app/src/main/kotlin/com/simplemobiletools/clock/helpers/Constants.kt

@@ -45,6 +45,7 @@ const val TAB_CLOCK = 0
 const val TAB_ALARM = 1
 const val TAB_STOPWATCH = 2
 const val TAB_TIMER = 3
+const val TIMER_ID = "timer_position"
 
 // stopwatch sorting
 const val SORT_BY_LAP = 1