Remove on-demand initialization of synthetic password

Now that all users are given a synthetic password, either at user
creation time by LockSettingsService.createNewUser() or at boot time by
LockSettingsService.onThirdPartyAppsStarted(), there is no need to
support on-demand initialization of the synthetic password when setting
the lockscreen credential or adding an escrow token.

The only issue is that the unit tests were relying on this.  This CL
addresses that issue by updating the unit tests to explicitly initialize
the synthetic password when needed.

This CL removes two TODOs from LockSettingsService.

Test: atest com.android.server.locksettings
Bug: 232452368
Change-Id: I7a1675953389ac1a261463e09291a2ff3b256483