MediaMetrics: Accept elided second argument in string pair

(device, address) string pairs used to have an optional second argument
that was empty.

Before:  (d1, )|(d2, a2)

we now permit elision of the second argument entirely, so that
can now be written as

After: d1|(d2, a2)

Note: the prior string continues to be accepted.

Flag: EXEMPT bugfix
Test: atest mediametrics_tests
Test: play ringtone with BT buds and check mediametrics dumpsys
Bug: 376948941
Change-Id: I41fd2f6acf1ea42cc5e683835790769148988b7d