Add measurements for TransportClient connections

Retrievable via 'adb shell dumpsys backup transportstats'.
Sample output:

Average connection time: 36.00 ms
Max connection time: 181 ms
Min connection time: 7 ms
Number of connections: 16
Per transport:
    com.google.android.gms/.backup.BackupTransportService
        Average connection time: 27.71 ms
        Max connection time: 139 ms
        Min connection time: 13 ms
        Number of connections: 14
    com.google.android.gms/.backup.component.D2dTransportService
        Average connection time: 181.00 ms
        Max connection time: 181 ms
        Min connection time: 181 ms
        Number of connections: 1
    android/com.android.internal.backup.LocalTransportService
        Average connection time: 7.00 ms
        Max connection time: 7 ms
        Min connection time: 7 ms
        Number of connections: 1

Bug: 72485465
Test: Will follow in another CL if reviewers OK w/ approach.
Change-Id: I133ed423d0b8471d69e3c3631aadee7d42d0ec0e