Use tidy_checks_as_errors not -warnings-as-errors

The flag -warnings-as-errors embedded in tidy_flags
is difficult to process and error-prone.
They should be replaced with the new tidy_checks_as_errors list.

Bug: 229801437
Test: make tidy-system-vold
Change-Id: I801e73dce5f08fe1ca5bc49de633706ad2cdf986