Fix flags linking for SystemUI

Targets that build against the platform (with no set SDK version) can
and should reuse flags that are already linked into the framework. Fix
this for SystemUI.

Bug: 326561340
Test: m + check duplicate library/program warnings from R8
Flag: EXEMPT bugfix
Change-Id: I15c02e8c4ffc762df1ce3249de77cdb15f94ae30
2 files changed