[SettingsProvider] fix SettingsProvider on work profile restart

A recent CL ag/25164661 changed SettingsProvider from lazily initializing to promptly initializing all the tables on-user-added. The old implementation of SettingsProvider used to promply clear all the settings tables when a user is stopped. This no longer works if we initialize the tables when the user is created, resulting in NullPointerExceptions. In this CL, we no longer clear the settings tables when a user is stopped. Instead, we only clear the tables when the user is removed.

Change-Id: I6d64f7256e99e3519158e62768519107a423723e
BUG: 307323531
Test: manual; stop and start work profile and observe no crashes