Binding on-demand #3: updateTransportAttributes() API

This CL introduces the updateTransportAttributes() API to be used by the
transport hosts. It doesn't actually use the description attributes yet,
this will go in another CL. This is because I want to test that CL
together with transport usage. Tests are lacking for TransportManager
and BMS, I'll still add them, but I'm trying to migrate Robolectric
first.

Ref: http://go/br-binding-on-demand
Bug: 17140907
Test: make RunFrameworksServicesRoboTests
Test: runtest -p com.android.server.backup frameworks-services
Test: gts-tradefed run commandAndExit gts-dev -m GtsBackupTestCases
Test: gts-tradefed run commandAndExit gts-dev -m GtsBackupHostTestCases
Test: cts-tradefed run commandAndExit cts-dev -m CtsBackupTestCases
Change-Id: I56f7b5a5026d21d8f11afb371d5560d4913c5f2a