mpeg4/av1: Update the minimum size check for AV1 CSD

The minimum required size for the AV1C box is 4 (and not 5). The
specification can be found in [1]. The sum of all the mandatory
fields in the AV1C box will fit in 4 bytes (note that the "configObu"
entries are optional).

The similar check that we do in Utils.cpp [2] when parsing AV1C is
correct.

This bug was introduced in ag/6271779.

[1] https://aomediacodec.github.io/av1-isobmff/#av1codecconfigurationbox-syntax
[2] https://cs.android.com/android/platform/superproject/+/master:frameworks/av/media/libstagefright/Utils.cpp;l=644

Bug: 141654151
Test: AV1 media cts tests still pass
Change-Id: Iadc9a1b643d55ac88dd00c4e0face05f464e8d05