ActivityManager: Add link/unlinkToDeath

Native services aren't killed when the system service dies, so they
may need to be able to tell when ActivityManager has died.

Test: adb shell stop; adb shell start; verify camera service still
   allows camera use. No regression to camera CTS.
Bug: 74230547

Change-Id: Iaa078b2f9a1d9c7be305b7667e3055a8ce52aa21