SystemServer: stop checking for CryptKeeper mode

Since support for Full Disk Encryption was removed in favor of
File Based Encryption, "CryptKeeper mode" no longer exists and the
"vold.decrypt" system property is never set.

system_server has the concept of running "core apps only", which is only
used in CryptKeeper mode.  This is now unused code, so as the first step
towards removing this functionality entirely, remove it from
SystemServer.java and hard-code coreApps=false for three services.
Later CLs will remove this functionality from these services.

Bug: 208476087
Change-Id: Ib14470dfea5e77e0bfd4406f0f31e3a96e8eff79