Add privacy hub top level pref and usage duration

Bug: 176902658

There's also these UI changes/polishes
1. Merge the feature flags: now only use privacy_hub_enabled
2. Add the today and yesterday tags
3. Make the time text width fixed (instead of sometimes wider sometimes not)
4. Correct the time format so that we always show the time instead of date
5. Show the duration usage information
6. Add the top level preference to view permissions usage

Test: The above demo on Sargo DVT
Change-Id: I08e23b7124e4dc5e0e075dcab6c6b1c16cb30203
7 files changed