Clamp brightness to dim value if we're timing out and playing the screen off animation.

This is the value PowerManager uses to dim the screen just before timeout, per https://source.corp.google.com/sc-dev/frameworks/base/core/res/res/values/config.xml;l=1349?q=config_screenBrightnessDim.

Fixes: 190581215
Test: watch the screen off, no brightness jump
Change-Id: I54856416dae1270c4379cb0547712edeba02b204