android container should not enable adb and mtp from default.prop

if the android container rootfs is built with ro.debuggable=1 it will put a default.prop file into the container rootfs with persist.sys.usb.config set to mtp,adb. on first boot after a factory flash (not using ubuntu-device-flash) when there was no value for this property set prior to starting the container this causes the gadget to be in adb mode without running adbd daemon. in his case the device will show up as "offline" in "adb devices" on a PC until developer mode was toggled once in the settings UI.

lxc-android-config needs to ship a pre-start.d snippet that removes this entry from default.prop. this way recovery boots can still benefit from ro.debuggable being set while adb behavior will not be affected in normal boots.

--- /var/lib/lxc/android/pre-start.d/20-no-default-prop-adb ---

#! /bin/sh

sed -i '/^persist.sys.usb.config=.*/d' $LXC_ROOTFS_PATH/default.prop

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