Use ExecutorService to replace CompletableFuture

CL[1] introduced starting input with CompletableFuture for improving IME
performance.

Since CompletableFuture is not recommanded way to use in system server side for
memory usage concern, we use java ExecutorService and Future to remove
this dependency.

[1]: I6aa4a664cfd0c86f75cee2457715317194bbe5e2
      e0172102

Fix: 147331480
Test: atest CtsInputMethodTestCases

Change-Id: I10391b834c33458c3e0ac846ab738e805d235c26