Make browser preference fragment valid

Subclasses of PreferenceActivity must override isValidFragment(String)
to verify that the Fragment class is valid. But
com.android.browser.BrowserPreferencesPage has not checked this.

Change-Id: I3650eff8a6d0547f14c6aff54425204d6c19f103
Always show the action bar overflow

As with native action bars.

Change-Id: I460ee02b370d7f220a8702d2e7644a7c775e3b21
Reimplement titlebar autohiding to be compatible with chromium webview.

Patch Set 4: Fix code formatting

Change-Id: I4de873f7c089d4f7dede944d05f48d03000f3d99
Browser: Materialize

-Material Ui theme (green)
-New icon
-xxhdpi drawables

Change-Id: I291f4bba1e412a8e762666db0684b95105dcf8b0