Wait for IME animation to end before calling IMM.

Due to b/221483132, IMM#showSoftInput() may not work
while IME is animating.

NUM_TEST_ITERATIONS is reduced to avoid test timeout.

Bug: 220385622
Test: atest InputMethodStressTest
Change-Id: If2a9d6e88b801faf491141f5f702b735a490a8db