add indicator for the grid map

1. Flash green when asking for user to place ship on map
2. Flash red when the ship cannot fit in that spot
fix a bug regarding HP in single player mode

In single player, the player has HP.
HP is added when a ship is added onto the board.
However, when a ship is taken off the board (long press), the hp doesn't
get subtracted.
The fix here is to add a setter for the HP, then set the value of the HP
when a ship gets removed.