Access IMMS#mSettings only inside the lock

This is a preparation CL before addresssing Bug 35361128.

IMMS.MyPackageMonitor#isChangingPackagesOfCurrentUser() needs to be
called after taking a lock for IMMS#mMethodMap because it internally
relies on InputMethodManagerService#mSettings.

Test: Made sure that the device can boot and the IME is available.
Bug: 35361128
Change-Id: I66752a275f20f6ba33bc211ffb8d6fd269250c0d