Avoid binder call on getAlwaysOn()

This method might be called on the UI thread,
we should cache the value and only read it
back when it changes.

Change-Id: Iee34e7cd6da133c603700d1834097d72de1dfb10
Fixes: 70857288
Test: enable/disable AoD from settings
Test: runtest systemui-jank -c android.platform.systemui.tests.jank.SystemUiJankTests -m testLaunchSettings