Move backup of @pm@ into its own state.

Previously it shared the INITIAL state with various set up work and
initializing the transport if needed. By moving it to its own state this
will mean we can retry a non-incremental backup of @pm@ if the transport
indicates that it has lost the state.

I removed the "mStatus == BackupTransport.TRANSPORT_NOT_INITIALIZED" as
the mStatus could never have this value at that point.
TRANSPORT_NOT_INITIALIZED is returned by the transport.performBackup(),
which is not executed until after beginBackup() has returned.

Bug: 71887023
Test: Manually tested init case, incremental case, @pm@ error case, init
error case. There are no unit tests for this file, and it's 1200 lines
long which makes writing them for IC difficult.

Change-Id: I23b7512391068316f4c1039faf88cc27905f4492