AppOps: Add support for whitelist of packages.

By default AppOps gives MODE_ASK permission to all strict operations.
This can degrade user experience for pre-installed apps.

This change adds support for passing a whitelist XML file to AppOps.
Operations requested by packages listed in whitelist file will be set
to MODE_ALLOWED by default.

User can still go into settings and override permissions of these
operations.

Whitelist XML file should have following format:
Example: whitelist_appops.xml

<whitelist-pkgs>
    <pkg name="com.android.settings"/>
    <pkg name="com.android.systemui"/>
    .
    .
    .
</whitelist-pkgs>

Change-Id: I78bf5e7baa95f0e86f9856fb38bf1ca238691303