Pass null looper if the looper is not prepared in the calling thread

**Root cause**
The PreferenceController can be constructed by the
SettingsSearchIndexablesProvider where the looper of the thread is not
prepared. This result in not able to construct the PreferenceController
that will be used as part of the SettingsSearch.

Currently, if the SettingsSearchIndexablesProvider is not able to
construct the PreferenceController defined in xml, it would just
silently failed.

Test: atest SettingsUnitTests
Test: atest SettingsRoboTests

Flag: EXEMPT low risk bugfix
Bug: 352622249
Change-Id: I72a4ce24ec6842b9efe067e3cb7d1c73cd98a566