Add adb keys to debuggable AOSP builds

These don't exist on AOSP itself, but if they're added in, debuggable
builds will include the keys. This way automated test farms don't need
manual intervention to authenticate to the device over adb, but we don't
disable security for everyone else.

Bug: 32891559
Test: lunch aosp_marlin-userdebug; m
Change-Id: I59011a5c1f45b0c8bd5c2355d2b4c3671f5da4e4