Add support for extension version in variables

This makes the sdk extension version usable in genrule cmds via %d.

Bug: 195281582
Test: current_sdkinfo module in packages/modules/common.
Change-Id: I82a90b83d23b0ba3779a943c2ad444bf5cf937da
(cherry picked from commit e720c41f67a7e9309f1e6e48d5975383be7b2d28)