Error on conflicting <uses-permission>

Don't install packages where we have multiple, conflicting
<uses-permission> lines for the same permission.

For example, a package which contains:

<uses-permission android:name="android.permission.INTERNET" android:required="false" />
<uses-permission android:name="android.permission.INTERNET" android:required="true" />

will now fail to install.

In addition, this change slightly refactors the code, and creates a new
parseUsesPermission() method.

Change-Id: I0f4bb8b51dc4a0c5b73458a70f706e19829275d2