Fix ModifierShortcutManagerTests on AOSP build.

Stub PackageManager.resolveActivity() in the tests to always return
an activity, even if there isn't something installed that could handle
it. This makes the test independent of the state of the device which
may have some typical apps for handling the shortcuts missing.

Fixes: 356277860
Flag: TEST_ONLY
Test: atest WmTests:com.android.server.policy.ModifierShortcutManagerTests
Change-Id: Ie7d41a29f33d4c095f71472327523ddbe2bb7efe