Fallback: Do not merge time units from different languages

The fallback logic should not merge the time units from languages in
fallback chain. It should use time units from a fallback language only
when time units not defined.

For example, zh-hans 22 time units defined. Since zh-hans does not
have any plural forms(it has just 'other' form as per CLDR), time units
contain only -other forms(total 22).  The existing fallback logic merge
this set with English, which has *-one and *-other plural forms. Effectively
every languages has time units array of length same as English.

This is wrong because a time unit like:
  [day-future-one] => in {0} day
does not make sense for zh-hans. For day future CLDR defines only this timeunit
for zh-hans:
    [day-future-other] => {0}天后

See I50e28f601944955ac85e0a847941d7b4824e51e3

Change-Id: Id1fb340c5f5e6f100e5227d0dfdb4a3001d8fb9e