Set system property for FULL bugreport instead of default.

The addition of DEFAULT mode in dumpstate service requires FULL bugreport to be distinguished from DEFAULT as the system property is empty in both the cases, but the effective arguments are different.

Test: Full bugreport is being generated as usual from the device:
      1. Settings->developer options -> Take a bug report -> Full
      report.
      2. Enable bugreport shortcut from the settings. Long press power
      button and then long press 'Take a bug report' button.
      3. Long press power, volume up and volume down buttons.

Bug: 119877616
Change-Id: I07bc279221ac9fc03967b887ee511f276f0ec040