BatteryIcon: fix set color when using color on low battery

[ljahn]

As it currently is, the BatteryIcon can change to orange for low battery (if selected at creation). However, the code assumes the default color to be white. In my Star Trek watch face, the default Icon color is black.

This change makes it such that when recharging after getting orange (or even red) the icon returns to the color set by SetColor.

Also adds a convenience function to set the visibility of the icon.

[github-actions[bot]]

Build size and comparison to main:

Section	Size	Difference
text	372832B	16B
data	948B	0B
bss	22536B	0B