SF: pass acquire fence on BLAST callbacks

When latching unsignaled buffers, the acquire fence is
not signaled by the time BLAST callback is invoked. In
that case pass a fence instead. For latch signaled, we still
pass the acquire time itself to avoid sending file descriptors
over binder.

Bug: 198190384
Test: SF unit tests
Change-Id: Ic7ad9b603b60dbf46a62eaf6b76bfbdeeebf6cec