Add BackupEncryptionService which provides an IntermediateEncryptingTransport

BackupEncryptionService (which is in a separate APK) provides a IntermediateTransport
which binds to the real transport.

We do this so that IntermediateEncryptingTransport can encrypt (decrypt) the
data when sending it (receiving it) from the real transport.
In this CL, IntermediateTransport does not do any encryption. It simply forwards
the data on. We will add the actual encryption in the next CL.

Bug: 139269919

Test: atest -v BackupEncryptionUnitTests
Test: atest -v RunBackupFrameworksServicesRoboTests
Test: atest -v $(find frameworks/base/services/tests/servicestests/src/com/android/server/backup -name '\''*Test.java'\'')'
Test: atest -v CtsBackupTestCases CtsBackupHostTestCases
Change-Id: I83a84ed3e638eb1dd165fffe406495f80ff930aa