Add FLAG_INCREMENTAL and FLAG_NON_INCREMENTAL to performBackup().

These indicate if backup manager passed any previous state to the backup
agent when building a backup. The transport will use these to decide
whether to clear any existing encryption related state for the app.

I left the new flags as @hide for now.

Bug: b/71792427
Test: Tested manually with a buid of GmsCore updated to process the new
flag. I am attempting to write a robolectric test.

Change-Id: Ica0f6f578cd7d1ad29af532a45a2ee6685c84747