[sf] Fix latch unsignaled issues

With LatchUnsignaledConfig::AutoSingleLayer flag, we should only
apply a transaction with an unsignaled fence if its the only transaction.
If we pass an unsignaled fence to HWC, HWC might miss presenting
the frame if the fence does not fire in time. If we apply another
transaction we may penalize the other transaction unfairly.

With LatchUnsignaledConfig::Always flag, respect backpressure flag
and take the opportunity to simplify the logic by treating the flag as
a signal to ignore the fence when applying transactions.

Test: surfaceflinger unit tests
Bug: 276229937
Change-Id: Ib5cd8205d62fdf8912b7f5c2cad312a01cae4300