Added support for DO_NOT_ASK_CREDENTIALS_ON_BOOT DPM flag

When DO_NOT_ASK_CREDENTIALS_ON_BOOT flag is set in
DevicePolicyManager, the Settings UI:
1) Should not encrypt the device with password when the
   user encrypts the device for the first time. The default
   encryption type should be used instead.
2) Should not give the choice to the user whether to
   encrypt the device with password/PIN or not but always
   encrypt the device without password.

Related CL: https://googleplex-android-review.git.corp.google.com/#/c/665371/

Change-Id: Ic09f02c033a0b16b7ffc45bf6d675b62d1be4bd8