SystemUI: RAM bar view optimizations

findViewById() was being called every time updateSettings()
was being called. Instead, let's instantiate them once, and
remove calls to updateSettings() since it will be called
on its own when the setting values change.

Change-Id: Iaa02343caccb615921219be2a56b3bac4f3263b9
Signed-off-by: Roman Birg <romanbirg@gmail.com>