Add test helper to remove a local log

The static sLocalLogs map can cause state leakage between tests, leading to flaky failures.
This can happen when a test adds a log but does not clean it up, affecting subsequent tests.

Bug: 429287189
Flag: TEST_ONLY
Test: atest TelephonyBackupAgentTest
Change-Id: I47e44f242aafe7d3ec6d8a646b70945ba8a72a47