aconfig: move unit tests for is_valid_{name,package,container}_ident to aconfig_protos

Commit 277e5dc3 extracted the aconfig proto code from the aconfig
package to the new aconfig_protos package, including the
is_valid_name_ident, is_valid_package_ident and is_valid_container_ident
functions. However, the unit tests for those functions were left in the
aconfig package.

Code and their unit tests should be in the same Rust module. Migrate the
overlooked unit tests accordingly.

Bug: none
Test: atest aconfig.test aconfig_protos.test
Change-Id: I4a37e409ee6a17725a05dbab0cfb657841854e25