Connectivity metrics: add transports pretty printing

This patch also
  - partially reverts commit f927f0c5
    that exposed a getTransports method on NetworkCapabilities.
  - moves enumerateBits to BitUtils (as unpackBits), and adds the
    reverse packBit method.

Bug: 34901696
Test: manually looked at $ adb shell dumpsys connmetrics list
Change-Id: Id04f9080e7f75608deeb49306aec34941e71794c