I don't think running an Android layer on Fuchsia would necessarily be a problem for battery life (it's not like you'd need to emulate a different CPU), althoug

I don't think running an Android layer on Fuchsia would necessarily be a problem for battery life (it's not like you'd need to emulate a different CPU), although loading all the Android frameworks and services could be a RAM hit. And apps with native code excessively tied to Linux might be problematic on the Fuchsia kernel. Some Chromebooks can now run Android apps; I wonder how that works out for the user in practice. (I haven't tried myself.)