Reduce flakiness in IME Swithcer menu test

This changes the final assertion of
testImeSwitcherMenu_openLanguageSettings from verifing once to retrying
a few times until passing. When opening the Language Settings the menu
will be dismissed, but there might be a delay between this and the first
time we check the assertion.

Flag: EXEMPT testfix
Bug: 421359220
Test: atest InputMethodServiceTest#testImeSwitcherMenu_openLanguageSettings
Change-Id: I004fac212ed8c0e90291f0890d53da21a9c9692f