Start using InputMethodSettingsRepository

With this CL we start taking advantage of

  InputMethodSettingsRepository,

which enables us to get a cached InputMethodSettings instead of
dynamically querying available IMEs.

To keep the CL as minimum as possible,

  InputMethodManagerService#mSettings

remains unchanged. It will be removed in a subsequent bug 329163064.

There must be no user observable behavior change.

Bug: 329163064
Fix: 309837937
Test: presubmit
Change-Id: Ie2d3b7e9b75790f64789d0c2e96ae46b1dee3a1e