get launchable activity name of package from adb

You don't need root to pull the apk files from /data/app. Sure, you might not have permissions to list the contents of that directory, but you can find the file locations of APKs with:

adb shell pm list packages -f

Then you can use adb pull:

adb pull <APK path from previous command>

and then aapt to get the information you want:

aapt dump badging <pulledfile.apk>

Since Android 7.0 you can use adb shell cmd package resolve-activity command to get the default activity of an installed app like this:

adb shell "cmd package resolve-activity --brief com.google.android.calculator | tail -n 1"
com.google.android.calculator/com.android.calculator2.Calculator

$ adb shell pm dump PACKAGE_NAME | grep -A 1 MAIN