Prevent clickjacking attack in DocsUi.

* Added permission `HIDE_OVERLAY_WINDOWS` in the Manifest.
* Set the flag to hide overlay windows to true in BaseActivity and
  ConfirmFragment.

Bug: 233605527
Test: Manually, see http://b/233605527#comment4
Flag: EXEMPT bugfix
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:02b1e4f9df47c4cfc09bc0436de40e6d5e4b2c20)
Merged-In: I511730856be58cad3e13fa50bfac1e1ee2f5fee0
Change-Id: I511730856be58cad3e13fa50bfac1e1ee2f5fee0