Code refactoring: split installPackagesLI to some methods

This refactoring will change the locking behavior of mInstallLock.
Before: lock, prepare, scan, reconcile, commit, unlock.
After: lock, prepare, unlock, lock, scan, unlock, lock, reconcile,
       unlock, lock, commit, unlock.
Also will have followed up changes to remove lock for the prepare
and scan stages.

Bug: 321139675
Flag: EXEMPT Code refactoring
Test: atest CtsInstallHostTestCases
Change-Id: I18b9698226458fe218f34b10d973648f26c6f50b