UserSystemPackageInstaller only (un)installs when appropriate

The UserSystemPackageInstaller (USPI) uninstalls system
packages that are not needed, depending on the user type.
When that determination changes (or the feature is disabled)
it can also re-install those packages.
This cl specifies when it is appropriate for USPI to actually
perform the (un)installation.

1.
Introduces uninstallReason: records the reason why a package
was uninstalled from a given user. Right now, the only values
are UNKNOWN and USER_TYPE. The latter indicates that the USPI
system uninstalled the package. If the USPI whitelist changes
(or the USPI feature is disabled), uninstalled packages will
only be installed by USPI if the uninstallReason was USER_TYPE.
Any further uninstalls (e.g. by ManagedProvisioning) will reset
the uninstallReason to UNKNOWN, so USPI will no longer install
such packages in the future.
This prevents USPI from reinstalling system packages that other
mechanisms (such as ManagedProvisioning) uninstalled.

2.
USPI will uninstall a system package if it is blacklisted, but
only if that system package is new, i.e in two circumstances:
a. on first boot
b. on an OTA where the package was not present prior to the OTA

Bug: 143200798
Test: atest UserSystemPackageInstallerTest
Test: Confirmed (un)installations during manually simulated OTAs
Change-Id: Ia0714d1faa8f7c79082f2cc93a92ae36b9a4c918
Merged-In: Ia0714d1faa8f7c79082f2cc93a92ae36b9a4c918