SoftAVCEnc: Stop using the u4_strd field in the set dimensions struct

Since 90a3904fd3 in external/libavc, this field isn't used any longer.
After SoftAVCEnc stops using it, it can be removed from the library
altogether.

Bug: 22860270

Change-Id: I0e0fcab6740ef420df7776ede11b35aa1a383a48