Consolidate the language-switching logic

This CL does not change the existing behavior.
In I84291fd4a7d6192b, the IME rotation group is logically
devided into two groups, one is for IMEs that are declared as
supportsSwitchingToNextInputMethod == true, and the other is
IMEs that are not declared so. The problem is that the logic
was implemented with a single ime/subtype list where these two
kinds of IMEs are completely mixed. This makes the code
unnecessarily complex.
With this CL, these two rotation groups are actually managed
as two different collections separately. This allows us to
simplify the rotation logic as well as its test cases.
This CL is also a groundwork to implement smarter
language-switching logic that is applied to
language-switching-aware IMEs only.

BUG: 7043015
Change-Id: I7f08ec299ec41d614e2cd3912320687db1576e80