Cache value of shouldUseSplitNotificationShade

This only changes when the flag value changes (which can happen at any
time, but is strictly for development so we can ignore) or the value of
the boolean resource changes, which only happens on configuration
changed. Instead of checking every time, we can cache the last known
value and update on configuration changes.

Bug: 182254944
Test: manual + atest SystemUITests
Change-Id: Iae54b7680ebfc84c48139bd21e3c7670739802a5