Unlink death recipient when clearing up SignalStrengthUpdateRequest

When SignalStrenthUpdateRequest was set, a IBinder death recipient
was linked to monitor the client's liveness. The same recipient
should be unlinked when the request was cleared. Otherwise it will
cause IBinder object leak and multiple binderDied() invocations
if the same request was set/cleared multiple times.

Bug: 213406784
Test: atest SignalStrengthControllerTest
Test: go/signalstrength-s-ft as regression test
Change-Id: If0f9bdb03283abef3864a3be51f61ea3b8baef4b