Remove FDE support from accessibility settings

With FDE (Full Disk Encryption), secure start-up (i.e. requiring a PIN /
pattern / password to boot the device) was incompatible with
accessibility services.  Thus, the accessibility settings would ask the
user to disable secure start-up when enabling an accessibility service.

Now that FDE support has been removed in favor of FBE (File Based
Encryption), this is no longer necessary.  Remove it.

Bug: 208476087
Change-Id: I5f6e512f223df63e1b4d1c181fc8b3fe683dcd5f
(cherry picked from commit b4060ef6)
Merged-In: I5f6e512f223df63e1b4d1c181fc8b3fe683dcd5f