Fix nullability issues in SettingsProxy classes

Some methods had wrong nullability annotations which didn't fit documented behaviour. This fixes those cases and prevents errors in IDE.

Bug: 327558308
Flag: EXEMPT only typehint changes
Test: SettingsProxyTest and UserSettingsProxyTest
Change-Id: Ida01f1072e7fc63373965f65cdc70ef58eb463c9
7 files changed