Calls unlinkToDeath in binderDied to deregister

When a registered BLE App unexpectedly dies and the binderDied callback
is called, the unlinkToDeath is called to remove linkage. Also, refactor
code to use an existing function.

Test: Manual test by repeatedly killing the gms core process.
Bug: 74076974
Change-Id: If47a534ecafe7fceae14f8cf8526987cabd279cd