Only set locale in config for updating locale

To update locale, it doesn't need a full configuration because
system only applied the diff of config from non-undefined fields.

IActivityManager#getConfiguration actually gets the configuration
according to the pid, which is process configuration. And process
configuration can be overridden by the activity running in its
process. So the returned value shouldn't be used to update the
system persistent configuration.

Therefore it is enough to use a new Configuration instance with
setting the necessary fields. This is also more efficient that
saves a binder call to system.

Bug: 253386061
Test: atest LocaleManagerTests
Change-Id: Icdce437fcf1a3bef0562cfc4dd5ad3ba52ea08ef