Docs: Add CURRENT_BRANCH variable and code.

Use a variable CURRENT_BRANCH to designate the AOSP branch to use for
the changelog.  Update code to make the substitution.

Bug: 34106192
Test: python make_cdd.py --version 7.1 --branch nougat-mr1-dev

Change-Id: Ia68946b91e61129929dd0dfb91d495d3c53b095b
2 files changed