releasetools: Prepend 'ota-' to 'streaming-property-files'.

'streaming-property-files' is a property related to the OTA package
itself. Prepend 'ota-' to make it consistent with others like
'ota-type' and 'ota-required-cache'.

Bug: 34852392
Test: Generate an A/B OTA package and check METADATA entry.
Change-Id: Ia681e6e19ff509e6da0d8718933b42aac997e1cf