Add optional permission support to isDisplayablePermission.
A permission is displayable if it's required, or was previously
granted to the app.
Currently, this change is a no-op. The package parser code
does not currently honor <uses-permission android:required="false"> in
the application's manifest, and assumes a permission is always required.
This change sets the ground for future optional permissions work.
Change-Id: I2ec4a49adbfab9980e116ed43354f16bdeaa301d
a5043ed7ea