Simplify PLATFORM_BASE_SDK_EXTENSION_VERSION

Make PLATFORM_BASE_SDK_EXTENSION_VERSION default to
PLATFORM_SDK_EXTENSION_VERSION: they only need to be different on the
SDK extension branches.

Bug: 247413701
Test: manual install of modules
Change-Id: Iedcd9d6bce4e7346d24146b871d489255a17466c
Merged-In: Iedcd9d6bce4e7346d24146b871d489255a17466c