Support generating sdk snapshot for specific build release

Previously, the sdk snapshot was assumed to be generated for the
current build system. This change adds support for the
SOONG_SDK_SNAPSHOT_TARGET_BUILD_RELEASE environment variable which can
be used to select the target build release in which the generated
snapshot will be used.

At the moment this will only affect the properties that are output but
if/when it becomes necessary it can also be used to control more
aspects of the snapshots such as supported member types or even
members.

This change does not modify any properties that are build release
specific. That will come in following changes. However, it does add a
test that targets build release S which defines a baseline for
properties that will be affected in following changes. That baseline
makes it easier to see the effect of those follow up changes.

Bug: 197842263
Test: m nothing
Change-Id: I5cfde5e9e86af522b0d5030d301e116b0233fd62
Merged-In: I5cfde5e9e86af522b0d5030d301e116b0233fd62
(cherry picked from commit ef2df587)