Stop trying to update FDE password from LockSettingsService

Since FDE is no longer supported, updating the FDE password never does
anything.  Stop trying to do so.  Remove updateEncryptionPassword() from
ILockSettings, since its only caller outside of LockSettingsService
itself was in LockPatternUtils, and the previous CL removed that caller.

Bug: 208476087
Change-Id: I46c2a472177836f0c9084e4c3b4ed2e6c0ab61d5