Exclude unused strings for chrome://ntp on Android

Relanding https://codereview.chromium.org/174453002 as it got reverted in https://codereview.chromium.org/177903009
and reverted again in https://codereview.chromium.org/196423006

I fixed the IDS_NEW_TAB_OTR_EXTENSIONS_MESSAGE string that had $2 placeholder
instead of $1 and caused the breakage:
build.chromium.org/p/chromium.win/builders/Win7%20Tests%20%28dbg%29%282%29/builds/20292/steps/browser_tests/logs/CookiesEventsSpanning

BUG=326023
TBR=estade@chromium.org

Review URL: https://codereview.chromium.org/198093002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@256707 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 256707