Simplify command in build-app/README.md

[dfaure]

The current approach is to run gradle once, then repackage the ARM version of aapt2 into the jar file, then rerun gradle. This is quite complicated.

I suggest the following instead:
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echo "android.aapt2FromMavenOverride=$ANDROID_HOME/build-tools/latest/aapt2" >> ~/.gradle/gradle.properties
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(adjust the path to aapt2 if needed)

Then ˋgradle buildˋ works out of the box on ARM64, from the first try.