TimeStats: make sure acquire fence is captured

Previously, if setAcquireFence happens earlier than setPostTime, the
acquire fence is not recorded in TimeStats which ends up with acquire
time being the same with post time. This issue is almost eliminated
after a previous refactoring of the order of recording TimeStats and
EGL throttling logic.

Since acquire fence is already available at post time, this change just
captures that by then to avoid any chance of missing the acquire fence.

Bug: 140266250
Test: atest libsurfaceflinger_unittest:TimeStatsTest
Change-Id: I90a5ab609afc7054c618273c3af79c9d12d85730