Additional subtype ID should be independent of OS-version

This CL consolidates the initialization logic for additional
subtypes so that each additional subtypes can have predictable
subtype ID regardless of OS version.

Previously subtype IDs for additional subtypes are calculated
differently depending on the running OS version with hoping it
minimizes the risk of compatibility issues.  However, it is
getting harder and harder to maintain slightly different
logic between OSes. Thus we decided to unify the logic into
that in KitKat even though it may causes some breaking changes.

Note that the actual extra values that are used to instantiate
InputMethodSubtype object are still determined on the fly
depending on the running OS version. However these actual
extra values are no longer used for the subtype ID calculation.

BUG: 16000850
Change-Id: Id3c262386a7bc7ed75966b1395a50171abe550d3