it absolutely will not be android. Guaranteed. Lets set aside for a moment that Nintendo rolls their own OSes. Android is built off of Linux, which has the GNU license, which is a recursive license that requires that everything built off of it uses said license. The GNU license requires that all changes made must be published publicly. Thus, if Nintendo put a modified Android on the Switch, The source code for their OS would be open to the public, and that is NOT gonna happen.
There's more to OSes than "Oh, that's a Tegra processor? Must run Android."