Runtime permissions cannot be set on legacy apps by device policy

Clarify docs that runtime permissions can be granted or revoked by
a profile owner/device owner only for MNC apps and not legacy apps.

Check the targetSdkVersion and return false if legacy app.

Remove all policy flags from permissions when cleaning up
a device or profile owner.

Bug: 21835304
Bug: 21889278
Change-Id: I4271394737990983449048d112a1830f9d0f2d78