Revert "Remove Incognito mode from quick controls since it no longer exists in the browser"

This reverts commit a2c4940fd5cf02bc97721bda8739df356baf0c83.

Change-Id: Ie66e3116477fc1a490c31cb6e810343909c9b217
Revert "disable incognito mode for now due of missing support"

This reverts commit ba23cb02b0f49c3f5650bfde2796066f0e7adf8f.

Change-Id: I25d91b14d1d97dcded26904454dda1d04aae497d
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
Show a startup page in incognito mode

Change-Id: Ib1d4be6890c81b73a46f026b6fcbb2f1e8de567d
blue to grey browser's progress bar

Change-Id: I7cabb70f84cd4f6c65a1c8b197831dad822498fa