BootReceiver: move to services/core/java/com/android/server

Per Jeff Sharkey's suggestion, move this file to another subtree to
allow using com.android.server.IoThread. No functional change.

Test: manual runs with KFENCE enabled
Bug: 172316664

Signed-off-by: Alexander Potapenko <glider@google.com>
Change-Id: I338283f17ba7c108d9bdd60b3e01d6bcbfc6b38a