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An android device is designed to save your credentials specifically so you don't have to enter them every time you need to access information. There are password manager apps that will offer some protection, but they are counter intuitive to the way Android (or any mobile platform) is supposed to work. The better solution would be to use a password or pattern lock on the phone and install security software that performs a remote wipe of your data. That way if the phone were stolen, too many incorrect patterns or passwords lock the phone and only entering your Google credentials or a factory reset will get the phone usable again.