Add copy-on-write mode to TextServicesSettings.

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

This is a preparation for File-Based Encryption (FBE) support in TSMS.

With this CL, we start switching TextServicesSettings from copy-on-write
mode to real mode and start rebuilding the list of available spell
checkers when the system becomes ready.  Anything else should be the
same.  See the previous commit message [1] for details.

  [1]: I9c6f9bb3d51174198e5f73588637f87ea0d90e11
       68645a63

Bug: 27456430
Change-Id: Ie3d61458648df469abe149b7aaad8087c531a675