Add a test mode to SecureSettingsWrapper

This is a preparation before moving

  InputMethodManagerService#mHardwareKeyboardShortcutController

to UserData so that each user can keep maintaining its own instance.

The issue is that currently UserDataRepositoryTest does not mock
anything around secure settings and UserManagerInternal, which means
that when it triggers logic that internally accesses to
SecureSettingsWrapper, it ends up throwing NullPointerException as

  LocalServices.getService(UserManagerInternal.class)

returns null, and even if we mocked UserManagerInternal, we still face
a problem on how SecureSettingsWrapper should be have when the unit
test passes a nonexistent user (e.g. 1).

As an intermediate solution, this CL introduces an official fake
implementation of SecureSettingsWrapper, which basically forwards all
the read/write operations to an on memory dictionary rather than
exposing the real data. This should be sufficient for now.

Bug: 305849394
Test: presubmit
Flag: TEST_ONLY
Change-Id: I7cdef44c32871d65d9d2ce2bcdf321cc912d9992