Implement a sketch of the new MD Incognito NTP on Android

This CL introduces:
- A feature flag for the new Incognito NTP on Android
- A new FrameLayout for this page (XML + Java)
- Reuses the HTML-formatted strings from the new Desktop Incognito NTP

The CL only contains structural changes; it is not yet styled to match
the spec (https://gallery.googleplex.com/projects/MCHbtQVoQ2HCZeKud1YOmPiI)

BUG=693525

Review-Url: https://codereview.chromium.org/2902013002
Cr-Commit-Position: refs/heads/master@{#474381}
diff --git a/chrome/browser/about_flags.cc b/chrome/browser/about_flags.cc
index a22acbed..65c1957 100644
--- a/chrome/browser/about_flags.cc
+++ b/chrome/browser/about_flags.cc
@@ -2212,7 +2212,7 @@
      SINGLE_VALUE_TYPE(switches::kEnableMaterialDesignFeedback)},
     {"enable-md-incognito-ntp",
      flag_descriptions::kMaterialDesignIncognitoNTPName,
-     flag_descriptions::kMaterialDesignIncognitoNTPDescription, kOsDesktop,
+     flag_descriptions::kMaterialDesignIncognitoNTPDescription, kOsAll,
      FEATURE_VALUE_TYPE(features::kMaterialDesignIncognitoNTP)},
     {"enable-md-settings", flag_descriptions::kEnableMaterialDesignSettingsName,
      flag_descriptions::kEnableMaterialDesignSettingsDescription, kOsDesktop,