Route backup data through IntermediateEncryptingTransport

The main change is in TransportManager which now uses
TransportClientManager.createEncryptingClientManager

We do this so that IntermediateEncryptingTransport can encrypt (decrypt) the
data when sending it (receiving it) from the real transport.

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: I01a0108da6b1f868ac6cb4591445ecc62316452f