Add a test to verify insets flicker on split screen

This is a regression test for bug 381134667.

The issue only happens when edge-to-edge is enabled, thus this adds
TestActivity to have option to call fitsSystemWindows with a given
argument.

Bug: 384892529
Test: ImeInsetsVisibilityTest#testImeInsetsChangesOnceInSplitScreenE2EApp
Test: the test fails without the previous regression fix
Flag: TEST_ONLY
Change-Id: I5c981188ce8bfa6c608e65b30bdb2ee60bee9137