Only set set_hashtree_disabled_flag in top-level vbmeta.img.

It's sufficient to have the flag in top-level vbmeta.img, and libavb
expects no disable flag in chained vbmeta.img.

Bug: 118115607
Test: `m vbmetaimage` with an -eng target using vbmeta_system.img. Check
      the flag in the generated image
      (`avbtool info_image --image vbmeta_system.img`).
Change-Id: Ia429dd8a5cf0243e16ae305da210d3ea27c68abf