Add FLAG_INCREMENTAL support to LocalTransport.

Implement performBackup() with flags argument:

1. If FLAG_INCREMENTAL is passed and transport has no data for this
package or supports only non-incremental backups, request a non-incremental retry.
2. If FLAG_NON_INCREMENTAL is passed, but transport has data, discard
that data and continue.

Bug: 72212840
Test: GTS test in the same topic
Change-Id: I4c4a1dbfa1a807e4d4c523f42bb6a404a39731e9