Implement client-side incognito mode

Since WebView deprecated privateBrowsing in API 17 (and killed it
for good in API 19), we were left without incognito in the Browser
app. Follow the docs' recommendations and implement it with "manual"
control of privacy-related engine options

* Internally track incognito state for tabs
* Disable all forms of storage and cache on private webviews
* Disable all cookie activity while an incognito tab is active
* Stop trying to use the deprecated "privateBrowsing" argument
  when creating webViews, always set to false

Change-Id: I23f2e34ee125635bba8981f0711ba4986a9beaab