Use ISO 8601 format for LocalLog local timestamps

Change to using ISO 8601 duration formatting for LocalLog local
timestamps, which makes it consistent with other time / time zone
detection dumpsys for easier comparison.

LocalLog is used by NetworkStack code as a standalone library, so
therefore has to depend on public SDK APIs, hence the use of Duration.

The local timestamp logging mode is only currently used for time / time
zone detection, which is low-frequency logging.

Bug: 204855374
Test: atest core/tests/coretests/src/android/util/LocalLogTest.java
Change-Id: Idce0d39c59670facdf48deca852b862ff0d71991