Report deltas as 0 instead of -1 for prediction expired display frames

Since both positive and negative values are valid for the deltas,
reporting -1 for cases where we cannot provide a delta skews the
metrics. Instead, report 0 as the delta since the jank is already
reported as unknown.

Bug: 184986864
Test: libsurfaceflinger_unittest
Change-Id: I09cbb0cb7084620038b4d5ada6c196647733bbe5