Firstly I used 7z to unzip the .apk file.

7z x candroid.apk

Then I checked strings to see if there was a flag somewhere. Surprisingly, there was.

strings resources.arsc | grep flag

flag{4ndr0id_1s_3asy}

A with CAndroid, I used 7z to unzip the .apk file.

7z x candroid.apk

Then I grepped all the files to see if there were any references to flag. If there were, I checked them out.

grep -rnw "flag"

This shows match in "classes.dex"

cat classes.dex | grep flag

flag{3asY_4ndr0id_r3vers1ng}