avoid assuming build number is a 32-bit integer

The install logging currently assumes that the build number is a 32-bit
integer and prints an error when that doesn't hold true. However, that's
only a convention used by Google builds.

From build/core/version_defaults.mk:

    ifeq "" "$(BUILD_NUMBER)"
      # BUILD_NUMBER should be set to the source control value that
      # represents the current state of the source code.  E.g., a
      # perforce changelist number or a git hash.  Can be an arbitrary string
      # (to allow for source control that uses something other than numbers),
      # but must be a single word and a valid file name.
      #
      # If no BUILD_NUMBER is set, create a useful "I am an engineering build
      # from this date/time" value.  Make it start with a non-digit so that
      # anyone trying to parse it as an integer will probably get "0".
      BUILD_NUMBER := eng.$(shell echo $${USER:0:6}).$(shell $(DATE) +%Y%m%d.%H%M%S)
    endif

Change-Id: I8e7cec0618783f69545ba76d0dce2bbc1681784c