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

Revert "Browser: inkognito mode is not supported in 4.4"

This reverts commit c424cb039aefa44891a097ae3bfbd5e7eeb3d9cc.

Show a startup page in incognito mode

Change-Id: Ib1d4be6890c81b73a46f026b6fcbb2f1e8de567d

This re-adds incognito mode back into stock browser properly for android 4.4 since webview caused the old implementation to force close the stock browser
credits to cyanogenmod
Implement client-side incognito mode http://review.cyanogenmod.org/#/c/57791/
Show a startup page in incognito mode http://review.cyanogenmod.org/#/c/58185/

Change-Id: Ib1d4be6890c81b73a46f026b6fcbb2f1e8de567d