Truncate vbmeta to 64KB

This is a porting of https://r.android.com/1675465 to make in order to
make the output filesize of the vbmeta partitions identical between make
and Soong.

Test: diff soong vs make vbmeta_system partitions
Bug: 390204058
Change-Id: Ieaf7583d4bb3cfe9da80f8abccf0af3e2372e638