Keyguard: Remove fancy colon

The new fonts do not have a dedicated fancy colon for the lockscreen clock
and fall back to Roboto which breaks the kerning.

Change-Id: I4621c6248de3f42f6c3c49ee4149bc1c0bb233e6
Fixes: 63891134
Test: visual; runtest -x packages/SystemUI/tests/src/com/android/keyguard/KeyguardClockAccessibilityDelegateTest.java