Remove unused InputMethodInfo#isEncryptionAware().

This logically reverts a previous CL [1], which added
InputMethodInfo#isEncryptionAware() for File-Based Encryption (FBE)
support, since it turns out that the method in question is unnecessary
to make InputMethodManagerService encryption-aware.

  [1]: Icf921fe3661eccf4a589b08b616d05decc561356
       69811a98

Bug: 26279466
Change-Id: Ia4884bf5922ad453d4b9e5e3c6f0d17b36dc205d