Build: Add PREVIEW_SDK_FINGERPRINT.

Based on ro.build.version.preview_sdk_fingerprint, which is
generated by the build system based on the contents of the
public / system API fingerprint. This value helps implement
finer grained targeting of packages to preview releases.

Test: atest BuildTest; manual
Bug: 120439642

Change-Id: Ib2c28d96ea0fa2ed9a6be4c05580c164e39cdd9f