Refactored LocalTransport to log DataTypeResult as a String.

You can check by running:

$ adb logcat LocalTransport *:s &
$ adb shell 'settings put secure backup_local_transport_parameters "dump_agent_results=true"'
$ adb shell bmgr transport com.android.localtransport/.LocalTransport
$ adb shell bmgr backupnow android
$ adb shell bmgr run

Test: atest FrameworksCoreTests --test-filter=".*DataTypeResult.*"
Test: manual verification with steps above

Bug: 394173116
Flag: EXEMPT changes on debugging components only

Change-Id: Ife98467d8169984c9e4f212260e3d18442812666