recovery: Add screensaver mode.

While it's waiting for user input, dim or turn off the backlight to
avoid OLED burn-in. The backlight brightness will be reduced after the
first timeout (default 120s), and then turned off after the second.
Pressing any key will take it back to the normal brightness. While the
display is off, the first key input will only turn on the backlight.

The most common case that triggers the screensaver is under text mode,
such as waiting for menu selection or viewing recovery logs.

This CL doesn't change the brightness while it's installing updates or
performing wipes under UI mode.

When it encounters any install error under UI mode (user builds):
 - If it's NOT USB connected, it will reboot automatically after the
   first timeout (same as before);
 - If it's USB connected, it will dim and turn off the display per the
   change in this CL.

Bug: 34077703
Test: Boot a device with the new recovery image. Wait for timeout.

Change-Id: I0c14907e60340a7f037adb6e464942d099ada08b