Auto enable usb adb if booting with debug ramdisk

If booting with debug ramdisk (ro.force.debuggable is true), override
the persistent property "persist.sys.usb.config" to always enable adb.
This ensures a consistent debug experience. The developer can expect
that after debug ramdisk (debug boot image) is flashed,
1. Device is rootable with (adb root)
2. adb is enabled automatically
3. no-auth (if $ADB_VENDOR_KEYS is used)

Bug: 223517900
Test: 1. Flash & boot user build, verify device doesn't have usb
  debugging enabled.
  2. Reboot to bootloader and flash vendor_boot-debug.img.
  3. Boot and verify usb debugging is automatically enabled.
  4. Enter Developer Options and manually turn off usb debugging.
  5. Reboot and verify that usb debugging is automatically enabled.
Change-Id: I729e961d645f2dd521ead8ca30b5cb8eb1d9f909