Onboarding redesign

[HanfeiChen]

This is a work in progress PR. Some work are ready to be reviewed, while there are WIP parts to be updated.

Changes included:

• Break onboarding preferences into four separate screens + one thank you screen.
• Implement enabling notifications on first preference screen.

Features to be added:

• Implement enabling location, importing location history and enabling analytics.
• Make sure that the settings screen (the slider switches) is consistent with the options selected during onboarding.