Exclude non-permission entries from getAllPermissions()

ScopedAccessLocalPreferences#getAllPermissions() could return null
entries if there's rootViewMode entry in the preference file -
com.android.documentsui_preferences.xml.

This CL excludes the non-permission entries from the result of
getAllPermissions().

Test: Manual
Bug: 77658075
Change-Id: Ife059da4956f76bd1af28b8dff9630470c3ba353
(cherry picked from commit a30ab91bd2f8a2ec121deb62b5ac18d810371a6c)