Using a dedicated phone as "hardware wallet"
Is a comparable level of security achievable with an Android device with full phone encryption?


Short answer: No. This doesn't mean an Android device is bad, there are less secure ways to store bitcoins. But it does have more attack vectors than a hardware wallet. 

You can add whatever encryption to the phone you'd like, but ultimately if that device can connect to the internet, there is risk that some malware could be loaded to it. A hardware wallet is not at risk of this type of attack, the secure element is kept isolated from the internet.

For example, if screen capture malware is loaded onto the Android device, and it snaps a screen grab of your seed phrase, the encryption methods won't matter at all. This sort of attack isn't possible on a hardware wallet.