MPEG4Extractor: Make code more clear

We were taking the exact same action whether or not the
'Present' bit was set.  So we make the code more clear
that this is what we're doing.  We also add logging,
in verbose logging only, if we get the unexpected case
of the 'Present' bit not being set.

Bug: 77714339
Test: TreeHugger
Change-Id: I58c5f5bb01e2b307b5943ae43c6d685cf96f5477