Change the package cache to a SparseArrayMap.

Using a SparseArrayMap will make it easier to get the
InstalledPackageInfo object associated with a specific app.

Bug: 243588926
Test: atest frameworks/base/services/tests/mockingservicestests/src/com/android/server/tare
Test: atest frameworks/base/services/tests/servicestests/src/com/android/server/tare
Change-Id: I400e2d9d3ec8b76a293856c44375bb1d302c530f