ConfirmationPrompt: Use default values for UIOptions

ConfirmationPrompt passes magnified and inverted options to the keystore
service. While gathering the accessibility_display_inversion_enabled
setting, the implementation would throw an exception if this setting was
never set by the user. This causes the font scaling property to be
ignored. This patch uses default values in case the system setting is
not set.

Test: Run CTSVerifier Protected Confirmation test with increased font
      size.

Change-Id: I03a3ef56209c73ca7d2b2527a5f145f744148e38