Copy the out/soong/build_number.txt file into sdk snapshots

This change adds the build_number.txt file into the sdk snapshot to
make it easier to debug issues arising in partner builds caused by the
snapshots.

The file is added to the root of the snapshot and is called:
    snapshot-creation-build-number.txt

Bug: 201295663
Test: m nothing
      packages/modules/common/build/mainline_modules_sdks.sh
      # Check the contents of various snapshots.
Change-Id: Ib3ac0cea5c9315cad0b9a3c25d88759097e37163