Add support to the update_current.py script to finalize an SDK.

This follows the steps under "[FINAL]" described in go/android-sdk-finalization,
and additionally adds three optional test libraries that were added after the
doc was last exercised.

Note that:
- The exact directory structure used here will change, see doc [1]. This script
  will get another update in master when the new structure is approved.
- The name update_current is perhaps a bit misleading now, but it's a convenient
  place since these directories are structured similarly to current (they will
  be more similar soon).

Example cmdline
$ ./update_current.py -f 28 4696330

Bug: 77525052
Test: ./update_current.py -f 99 4696330 && diff ../../frameworks/base/api/current.txt api/99.txt
Test: ./update_current.py -f 99 4696330 && diff ../../frameworks/base/api/system-current.txt system-api/99.txt
Test: ./update_current.py -p 4696330
Change-Id: I5f5f2de8bffa13dc69c7f0ba474ed1c2425dec85
Merged-In: If7c51402763ab70f4f853aa9d3a26e5a7285a41d
1 file changed