SettingsHomepageActivity: Get profile parent using UM.getProfileParent

In the headless system user mode(hsum) non-primary users can also have a profile.
The existing implementation in SettingsHomepageActivity always gets the primary user as the profile parent, refactored the implementation to get profile parent using UserManager.getProfileParent.

Get parent user id instead of primary user as profile parent

Bug: 267299459
Test: croot && make RunSettingsRoboTests -j40
ROBOTEST_FILTER=SettingsHomepageActivityTest

Change-Id: I36845eb5b1a09b0ff3d87c7d08b120c95f59783b