Add basic user onboarding experience.

[iccub]

Description

Make a new onboarding experience for iOS.
Product spec: https://docs.google.com/document/d/1osrM-ljPmGtuV0igBcbtrG1GVXthBs1AnVNODFFv9rQ/edit?ts=5cfed804&pli=1

Designs

iPhone

iPad

Animated transitions between screens

1. Loading --> Search engine screen
   
   higher res video version: loading.mp4.zip

2. General transitions between screens
   When the white panel changes heights between screens, the height change and content should be animated for a smooth transition, for example (this is on Android):
   

Animation breakdown: the content in the white panel fades out: start Opacity: 1, end Opacity: 1, duration 0.2s. The white bg resizes starting at T=0.1s to the appropriate height for the next screen, duration 0.3s. The illustration and new text content fades in starting at T=0.3s: start Opacity: 0, end Opacity: 1, duration: 0.4s.

Assets

Grab the assets:
Search engine screen PNGs
png assets.zip

Shields screen motion graphic
shields.zip

Abstract link for more CSS and plain text: https://share.goabstract.com/bdad6a90-59ac-4735-ba48-673024c9c22e?sha=7f7b11dedded4529e4ce533acba202c280094a17

[karenkliu]

@iccub Design specs added for you!

[srirambv]

Verification passed on iPhone XR with iOS 13.1 running 1.12(19.09.06.17)

• Verified onboarding flow is shown on a clean install
• Verified SE changes made in onboarding is retained
  iOS_Onboarding.zip

Verification PASSED on iPad Air 3rd Generation iOS 13.1 running 1.12 (19.09.10.18):

• Verified that the onboarding experience specified under Add basic user onboarding experience. #1416 (comment) works as expected
• Verified that tapping on Skip correctly closes the onboarding modal
• Ensured that the onboarding experience doesn't reappear when relaunching Brave
• Ensured that the selected SE becomes default for both Standard & Private Tabs
• Ensured that the animations are smooth and not janky

Verification passed on iPhone 7+ with iOS 12.4.1 running 1.12(19.09.13.06)

• Verified onboarding flow is shown on a clean install
• Verified SE changes made in onboarding is retained