Add a feature flag for home application improvements

Improvements:
1.Allow uninstallation of current home app if it is a non-system
app and/or there are other candidate apps available
2.Prevent clearing of launcher role intents even when candidate
list changes

Bug: 310801107
Test: presubmit
Change-Id: I1983ff4ae3494e4b6a5dd0322ea5a1eb1911a3ec