Correct tree printing logic to prevent crash

The previous iterative implementation for printing the session tree in
`printSessionTreeLegacy` contained flawed logic for calculating indentation
depth. This led to two issues:

1. A `java.lang.IllegalArgumentException` was thrown when the depth counter
   became negative.
2. For most tree structures, the logged output was incorrectly indented,
   making it difficult to read.

This change replaces the buggy method with a standard, recursive
implementation (`printSessionTree`) that properly handles indentation.
This resolves both the crash and the visual bug in the logs.

Flag: com.android.server.telecom.flags.fix_session_tree_logging
Fixes: 427861789
Test: new unit test
Change-Id: Icfd1cae9be02c5fec8387053dc78f26bc1d5ff08