[SettingsProvider] allow null values in validators

As discussed in b/165943447, null is an important indicator that a
sepecific Settings element is unset, and should be treated as valid
values that can be backed up.

We should remove the non-null restriction for most of the validators,
especially because SettingsBackupAgent already can handle null values.

BUG: 170131663
Test: atest SettingsValidatorsTest
Test: atest SettingsBackupAgentTest
Change-Id: Ie44a6de503d83716c63be5c8e7937f8c9eb928bc