[SettingsProvider] add NPE check for acquireProvider

In rare cases, SettingsProvider cannot be found. For example, when a user is being deleted. In such cases, we should still prevent the caller from crashing by adding a null check for the result of acquireProvider.

BUG: 401857515
FIXES: 401857515
Test: presubmit
FLAG: EXEMPT bug fix
Change-Id: I9b233a9cdaff77e970ae5ae9bb46ee16c1d4054c