Differentiate the default route with the device route

Default route denotes the route that will be selected by
SystemMediaRoute2Provider by default. While the device route denotes the
route that is currenctly active among the phone speaker and wired headsets.

Also, some variables and methods are renamed for readability.

Bug: 149796428
Test: pass MR2 tests
Change-Id: I9133bb106ce6b509147e5f837cc30f229a762088