Fix broken KeyguardMessageArea

Fixes a bug where the keyguard message area would reorder clears
after a new message was set, leading to the bouncer prompt reason
not showing.

Change-Id: I33001300d9175c521809cd4fdae5158269245c00
Fixes: 32306174
Test: runtest -x $T/frameworks/base/packages/SystemUI/tests/src/com/android/keyguard/KeyguardMessageAreaTest.java