Epic task for adding offline compilations UI to the Android app

Proposed design

Access wireframes showing the entire flow from Invision:
https://wikimedia.invisionapp.com/share/NQBXQZYMD

Also available on the Android Zeplin board, screens tagged under "Offline-compilations"

Tickets that reference UI from this task include:

• T163587: Create a new activity for managing compilations
• T163588: Create a page/fragment to show the Offline Library (which contains a list of all available article packs for download)
• T163589: Create a page/fragment to show downloaded/active compilations

• T163590: Make a list of downloadable compilations searchable
• T163591: Make downloaded/active compilations searchable

• T163594: Tap a content pack to show a detail screen about it
• T164385: Create an empty activity for showing content pack details
• T164386: Show the list contents
• T164760: Populate the metadata for all the different content packs we're going to be supporting

• T164764: Execute the download of a compilation

• T163595: Let the user know that they're offline
• T166563: Allow user to choose which offline compilation to use when >1 is available
• T166559: Show amount of storage used by Wikipedia app and compilations to users

• T163596: [EPIC] Provide users with more bandwidth usage control
• T166596: Provide data saving preferences