Wait for launcher state before proceeding next test step

Otherwise if scrollToAppIcon is called immediately, it could
interrupt the state that causes the test to be unable to drag
the icon into workspace.

Fix: 388510107
Flag: TEST_ONLY
Test: atest Launcher3Tests:ThemeIconsTest#testIconWithoutTheme
Change-Id: Ied77b3f8d2f19de18d054a80643f3d13c5641ac4