Store original media dimensions as additional header

Store original media dimensions as additional header

For storage repos that support headers (such as Swift), this will store the original
media dimensions as an extra custom header, X-Content-Dimensions.
The header is formatted to minimize its length when dealing with multipage
documents, by expressing the information as page ranges keyed by dimensions.

Example for a multipage documents with some pages of different sizes:
X-Content-Dimensions: 1903x899:1-9,11/1903x873:10

Example for a single page document:
X-Content-Dimensions: 800x600:1

Bug: T150741
Change-Id: Ic4c6a86557b3705cf75d074753e9ce2ee070a6df