Fix InputMethodSettings#getInt() for nonexistent key when copy-on-write is on

It turns out that InputMethodSettings#getInt() has always returned 0
instead of the specified default value when copy-on-write is on and
the specified key does not exist since the mechanism was introduced
to support direct-boot [1].

Most likely this would not have been a big issue, but let's fix it
just in case.

[1]: I9c6f9bb3d51174198e5f73588637f87ea0d90e11
     68645a63

Bug: 26279466
Test: atest CtsInputMethodTestCases CtsInputMethodServiceHostTestCases
Change-Id: If2abc0604372cf394105db9395706aaa0f570431