Allow uninstallation of current home app

Change the behavior to allow uninstallation of the current home app
if it is a non-system app and/or there are other candidate apps
available.

Bug: 131721576
Test: atest com.android.settings.applications.appinfo.AppButtonsPreferenceControllerTest
Test: atest com.android.settings.spa.app.appinfo.AppButtonRepositoryTest
Change-Id: I556966894240aaf91c0e6424dce514b6a35d1001