Fix flaky flicker test

Fixes flakes in the test switchAppByDoubleTapDivider by changing the way click events are sent in doubleTapDividerToSwitch().

Previously, this test relied on SystemClock.sleep() and UiObject2.click() to create a double-click action; however, in a minority of cases, this was failing -- the component was registering two spaced-out single clicks instead of a double-click.

Using UiAutomation to inject the clicks at precise times fixes the problem.

Fixes: 397395900
Flag: EXEMPT test changes only
Test: A local test run of 200+ iterations ended with no flakes (previously was failing reliably by 30-50 iterations)
Change-Id: I9399a54129b31e2f9692ff6027a33fc868249170