Re-introduce debug.sf.latch_unsignaled

This pretty much reverts this commit: Ia453eaf895554adbb2fa9c11c2bc8609e1037ea5.
Some devices may find this helpful to improve performance, but there is a risk of pipeline stall from misbehaving apps.

Bug: 173694115
Test: manual
Merged-In: I48c3095061dc106fa08d36b8ef17eccb04fb6eb1
Change-Id: I48c3095061dc106fa08d36b8ef17eccb04fb6eb1
4 files changed