fdsan: improve diagnostics.

Generate a tombstone instead of just a log message when warning. Also,
don't use async_safe_fatal_va_list when we're not calling abort, as
this will permanently set the abort message.

Test: bionic_unit_tests
Test: setprop debug.fdsan warn; crasher fdsan_file
Change-Id: I2fe271da9004c8fd1f50ad3d8280be254eeaf656