releasetools: Pack the offset/length for metadata.

For streaming OTAs, we will also need the info in the metadata entry
(META-INF/com/android/metadata). Compute and pack its offset/length
values into 'ota-streaming-property-files'.

Bug: 34986195
Test: Create an OTA package and check the offset/length values.

Change-Id: Id150700f2bc9bff02467cda9fe8927c8a374412a