Update SystemUI to use platform WeaklyReferencedCallback annotation

This annotation is slightly more general in terms of application, but
removes the additional complexity from SystemUI proguard rules.

Bug: 275175207
Test: m + presubmit + apk diff
Flag: EXEMPT build fix, minimal risk
Change-Id: I75d9cc699d3c3c896cbaeabfcec42a026b082726