Check for FEATURE_INPUT_METHODS instead of MULTI_CLIENT_IME_ENABLED

Checking for FEATURE_INPUT_METHODS before running MockIme tests is a
safer way rather than relying on explicit MULTI_CLIENT_IME_ENABLED
property, which was introduced in Ifa59295cf2a1f6cce9f4e0ec69b0a81a0348246d

Bug: 137203665
Test: 1. Verified MockIme tests runs on phones.
      2. Verified MockIme tests skip when FEATURE_INPUT_METHODS is false
         for platforms like Android auto that support MultiClientIme.

Change-Id: I48b37dde16b1e8adbad381a1ce00b60a63ecdfb4