Improve the handling of profile snapshot in installd

Force aggregation when we take a snapshot. This will ensure we capture all
available data.

Also, pass the boot image flag when taking a snapshot for the boot image
profile. It will ignore regular app profiles during aggregation (instead
of failing).

Test: installd tests
Bug: 139884006
Change-Id: I110fc6e2639c170c6ae0bab812b0e4145113d093