SystemUI: stop using IPackageManager::isOnlyCoreApps()

Due to the removal of support for Full Disk Encryption, this method
always returns false.  In preparation for removing it, stop calling it
from SystemUI.

Test: mma
Bug: 208476087
Change-Id: I5b9de5667780388d19c1b3addae2124c35400ed2
(cherry picked from commit ac256182)
Merged-In: I5b9de5667780388d19c1b3addae2124c35400ed2