Only log first input reader length

It's not feasible to reliably differentiate a different
segment parsing from a seek.

Bug: 158742256
Test: atest CtsMediaParserTestCases
Test: atest CtsMediaParserHostTestCases
Change-Id: I7d62f1c2b9334bf601f38ce915b0fafdc76fbec1
Merged-In: I7d62f1c2b9334bf601f38ce915b0fafdc76fbec1
(cherry picked from commit 2eee34d2)