releasetools: Add sdk and security patch level info to metadata.

The info comes from the build.prop file of the target build (thus no
backward compatibility concerns). OTA server and client can optionally
use these info to understand the expected behavior of an OTA package.

Bug: 72751683
Test: python -m unittest test_ota_from_target_files
Test: `m dist`, then check the metadata in the generated OTA package.
Change-Id: I5935f67684d2486bb5f00d67ce4bc756589a56ed