Latch Unsignaled when only a single layer is being updated.

Three conditions are introduced.
DISABLED (Default for now): This is when latch unsignaled is completely disabled.
AUTO: This is when we will latch for the single layer update. Further refinements will be done in b/200284381
ALWAYS: This will latch unsignaled no matter what the change is.

BUG: 198189193
Test: Did the manual test and
atest libsurfaceflinger_unittest
atest MockFence_test
atest libgui_test

Change-Id: I0c0b475ba4a093275fac23a986fc610ea462f73e