As of March 2015, how many Android phones have already shipped with Trustonic TEE support built in? How many phones out there already have Trustonic TEE built-in? How feasible it is to develop an Android app that would rely on Trustonic for securely storing secrets (cryptographic keys) in TEE?
Here is Feb 2015 article where it is self-claimed that Trustonic technology is embedded in over 400 million smart connected devices.
Looking back here are some older references ...
According to Trustonic's profile at GlobalPlatform.org, Trustonic technology has already shipped in over 250 million smart devices. Similar number being published by several sources around summer 2014 timeframe.
According to this Feb 2014 article, just Samsung alone has already shipped over 150 million of devices that use Trustonic ‹t™-base TEE technology.