Avoid duplicate map lookups in RemoteConnectionManager.

Use Map.computeIfAbsent or the result of get() calls to interact with
the map via a single access, avoiding duplicate lookups.

Test: atest cts/tests/tests/telecom/src/android/telecom/cts/RemoteConnectionTest.java
Bug: 274764512
Change-Id: I1e6b10381f0c9b408c4a734384e550dfaefc1f40