Support privileged vendor apps

Privileged apps can now be located in the vendor partition. This is
mainly to move SoC-dependent apks to the vendor partition so that the
system partition becomes more generic.

Like existing privileged apps in the system partition, the list of
privileged apps in the vendor partition and the permissions they are
using must be white-listed. The whitelist can be specified via
<privapp-permissions> tags in one of /vendor/etc/permissions/*.xml
files. Note: vendors can only white-list the apps in vendor partition,
but not the apps in system partition.

This change also introduces a new flag 'vendor-privileged' to the
permission protection level. It is used to expose platform-defined
permissions to the privileged vendor apps. If a platform permission does
not have this flag, it is not granted to vendor apps even when the app
is privileged and white-listed.

Bug: 35301609
Test: `mm` under frameworks/base/tests/privapp-permissions
adb sync && adb reboot
adb shell cmd package \
com.android.framework.permission.privapp.tests.vendor
shows that the app is installed.
android.permission.BIND_IMS_SERVICE is in the installed permissions list
android.permission.MANAGE_USE is not in the installed permissions list,
but is in the requested permissions list.

Change-Id: I196375aaaa9ea3a2ba15686ef08cf3f70ade7046