Add pixel scaling to ScreenDecorationUtils

SreenDecorationUtils was querying rounded_corner_radius directly using
static methods defined in RoundedCorners.java, and was not taking into
consideration the potential for physical pixel scaling.

This CL replicates the logic that exists in RoundedCorners.java and
ScreenDecorations.java to implement similar scaling in the static
getter.

Test: manual
Flag: NONE
Bug: 335357341
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:1bd662453b41f64bcafdb125aae73782bdc40a4c)
Merged-In: I3ce403510b3ca35970685fc97a4fbcedaeb79151
Change-Id: I3ce403510b3ca35970685fc97a4fbcedaeb79151