Start home activities on displays that are empty

During system boot, there could have several home activities
being sequentially launched (e.g. FallbackHome -> default home).
If a home activity doesn't support running on secondary displays,
we weren't sequentially start the next home activity even it
does support running on secondary displays.

Start home activities on displays that are empty (no activity)
whenever home changed.

Bug: 119844270
Test: atest ActivityManagerMultiDisplayTests
Change-Id: I78a4edd44212c47e20852f5de26f2c3a244f99de