Enforce min_sdk_version=29

`libincidentpriv_defaults` are the defaults for
`libincident*`, which includes `libincident_test`.
This test is run as part of the MTS suite, setting min_sdk_version
ensures backwards compatibility with devices Q and above.

Test: atest [<mod_name>...]
Bug: 156476221
Change-Id: I9e6656662ec7337bc56e973921b69b866b0daacb