Keep original vsync for target work duration calculation

During buffer stuffing recovery, the target work duration
should be calculated using the original vsync timestamp
indicated by frameTimeNanos. Before, the target work
duration was updated during buffer stuffing recovery using
the adjusted frame time (which has a single frame offset due
to the recovery method), which led to the target work duration
being reported as longer than it actually was. This subsequently
decreased CPU clock frequencies and led to more possible jank
when expensive frames needed to be rendered.

Bug: b/399256574
Test: manually inspect perfetto traces
Flag: EXEMPT, bug fix
Change-Id: I959cf50bb35e370d48cbdbfb321f09992ee3db29