Create our own Desktop Mode common assertions.

This is done by adding new common assertions in DesktopModeBaseTest,
removing inheritance from ICommonAssertions, and adding a way to exclude
common assertions by adding excludedTests set.

Fixes: 419252045
Test: atest OpenAppFromAllAppsFlickerTest
Flag: EXEMPT test infra work
Change-Id: I3ae6e55b21b575cdc360ba6232cb6ed9e97141e1