Add new make variables for extension SDK version

- PLATFORM_SDK_EXTENSION_VERSION is the sdk extension version of the
  tree
- PLATFORM_BASE_SDK_EXTENSION_VERSION is the sdk extension version that
  PLATFORM_SDK_VERSION ships with.

Also propagate PLATFORM_SDK_EXTENSION_VERSION to soong to that it can be
used in genrules.

Bug: 195281582
Test: via development/ CL in this topic
Merged-In: Ib078a922e9bd9bc6ae82ab74feaeb743703a64b8
Change-Id: Ib078a922e9bd9bc6ae82ab74feaeb743703a64b8