Remove redundant code in certificate pref controllers

mPreferenceKey in BasePreferenceController is set via the second
argument of the constructor and has a getter. It doesn't look necessary
to override the getter to return the key. The given one also looks
inconsistent.

Test: atest com.android.settings.security
Bug: 139173976
Bug: 112038744
Change-Id: I6e20b46675308f7dbb8f82f7e372bf94f21e4bed