Remove delay of alternate bouncer showing signal during dreams

The delay on the alternateBouncerShowing signal, introduced to address
scrim issue b/339817220, caused a noticeable lag in fingerprint
authentication, especially on phones. This change removes the delay to
improve perceived responsiveness when showing the alternate bouncer over
a dream.

Test: verified that the alternate bouncer appears over dream without a
      noticeable delay
Test: verified that the scrim issue does not resurface on tablets
Fix: 405383897
Bug: 339817220
Flag: EXEMPT bugfix
Change-Id: Iaaad637f85d9af43b947f7dc40e6ad063dc29024