Expose an update_engine API that verifies the given payload metadata.

android.os.UpdateEngine.verifyPayloadMetadata() allows a system updater
to verify a payload can be safely applied, by using its payload metadata
(usually a few hundred KiB only or even less). This would be useful in
improving update UX, since the updater can be smarter to avoid
installing an update that would fail to apply, or to fall back from an
incremental to full payload.

Bug: 65283633
Test: Build and flash walleye on device. Trigger a call to this API.
Change-Id: I82546f6b2a8f27cfd956cc1677556e7221cce5fd