Set up BuildNumberFile in soong.variables.

Necessary for some build steps. aapt2 is one example, although it still
has problems because part of its build logic is in make.

This isn't strictly required for any supported build target at the
moment, but the error when it fails can be confusing so let's add it
anyway.

Test: build/soong/scripts/build-aml-prebuilts.sh art-module-sdk \
  art-module-test-exports conscrypt-module-sdk \
  conscrypt-module-test-exports conscrypt-module-host-exports \
  runtime-module-sdk runtime-module-host-exports \
  i18n-module-test-exports i18n-module-sdk platform-mainline-sdk \
  platform-mainline-test-exports
Bug: 172548608
Change-Id: I88526a85f288287c3a138f8cbb6fd04ad28c3795