AVR LED: Don't set brightness in LPM

A1 AVR is handing LED brightness change in LPM a bit broken. When brightness
gets changed it turns on the LED, notices it is in LPM and turns it back off.
This patch fixes the issue by forbidding brightness change in LPM. Fixes #13.
AVR: Don't update when values are not changed

Every I2C call to AVR takes about 1ms. When Android changes the brightness, it
changes it gradually e.g. from 255 to 128. AVR LEDs have 20 levels of brightness
and there are 12 times more updates than needed.

This fix caches the values and clears the cache when entering/leaving LPM.