Add new permission KEEP_UNINSTALLED_PACKAGES

Created a new permission KEEP_UNINSTALLED_PACKAGES and
granted it to shell, this new permission will be used to expose
PM#setKeepUninstalledPackages as a testAPI, this is needed
to enable the new testing infrastructure to reinstall
removed packages between test runs as part of the state resetting.

Bug: 179682115
Test: atest PermissionPolicyTest#platformPermissionPolicyIsUnaltered
Merged-In: I2b262f3694478631d657cdbf088b6511072d4ff3
Change-Id: I2b262f3694478631d657cdbf088b6511072d4ff3