Allow permissions to be granted via whitelisted signatures

When an application defines a permission, it can now add a new
attribute "allowViaWhitelist", a boolean value. If set to true,
the permission may be granted to a package signed with a predefined key,
if it is defined via <allow-permission> in
/system/etc/permissions/someapp.xml.

Since this is a hidden attribute, it must use the prv namespace XML
declaration. E.g.: add the following to the <manifest> tag:

       xmlns:androidprv="http://schemas.android.com/apk/prv/res/android

"

In the permission declaration:

        <permission
            android:name=""
            android:protectionLevel="signature"
            androidprv:allowViaWhitelist"true" />

And a corresponding entry in /system/etc/permissions/someapp.xml:

        <allow-permission
            name="some.android.PERMISSION"
            signature="<known public signature>" />

Note: if the permission never declares "allowViaWhitelist", then the
whitelisted permissions will be ignored.

Change-Id: Ie4597a07eb0a193375fa2724bd9cf468184a7926
Signed-off-by: Roman Birg <roman@cyngn.com>