Have unified setter/getter for TSMS Settings.

This ports my previous CL for InputMethodManagerService [1] to
TextServicesManagerService (TSMS).

In order to make TSMS encryption-aware, we are going to introduce a new
state where any read/write access to Secure Settings from TSMS is
virtualized so that we can temporarily enable only encryption-aware
spell checkers until the user unlocks the device then revert
any changes made before the device enters into an unlocked state.

See my previous commit message [1] for details.

This is still a preparation code.  Behavior change is not intended yet
in this CL.

  [1]: I0f79243e5cc1556764da37fa38078e075a27d42b
       87523670

Bug: 27456430
Change-Id: I97a1e2f75bdecb0aa88fb35dfe56732c08fcbbd5