Produce ro.build.version.preview_sdk_fingerprint

This is just a hash of the current.txt and system-current.txt right now,
though it may expand to include other API surfaces in the future.

Once prebuilts/sdk/current is populated with api_fingerprint.txt files,
we can use those for unbundled builds, but until then, just use the
PLATFORM_PREVIEW_SDK_VERSION as a placeholder.

MD5 was just the most convenient, since we don't have a sha tool that we
can use on Mac currently. I'm hoping we can get a toybox-based tool in
the future that standardizes that, but we aren't currently using sha*sum
from toybox on Linux, much less on Darwin yet.

Test: m dist out/dist/api_fingerprint.txt
Test: m out/target/product/.../system/build.prop
Change-Id: If69f270560d05135cb81a9bb2d1b208ea78f86df