Minimize the number of default enabled IMEs part 4

This is a follow up CL for recent attempt to minimize
the number of default enabled IMEs.
- part1: I831502db502f4073c9c2f50ce7705a4e45e2e1e3
- part2: Ife93d909fb8a24471c425c903e2b7048826e17a3
- part3: I6571d464a46453934f0a8f5e79018a67a9a3c845

It turned out that the changes made in part2 and part3 are
a bit overkill, and users will see no software keyboards
in some particular situations. The problem we missed in
the previous CLs is the fact that
InputMethodInfo#isDefault is indeed a locale-dependent
value, hence it may vary depending on the system locale.
Existing unittests also failed to abstract such
locale-dependent nature.

In order to addresses that regression, the selection logic
is a bit widely reorganized in this CL.  Now the logic is
implemented as a series of fallback rules.

Also, unit tests are updated to be able to 1) test the
order of the enabled IMEs, and 2) emulate the
locale-dependent behavior of InputMethodInfo#isDefault
to enrich test cases.

BUG: 17347871
BUG: 18192576
Change-Id: I871ccda787eb0f1099ba3574356c1da4b33681f3