Adding adb_debug.prop

The adb_debug.prop will be loaded by init when the file
/force_debuggable is present in the first-stage ramdisk, and the device
is unlocked. This file has the highest priority to override other system
properties, thus can override ro.debuggable to 1 to allow adb root.

Bug: 126493225
Test: boot a device with /force_debuggable in ramdisk, checks
      adb_debug.prop is loaded

Change-Id: I75d661bb04178f584d8b6f5eba26ae35ccb01239