Use Xcode-provided metadata for license accept

[timsutton]

Scanning the License.rtf file for text matching the agreement as is done here seemed a bit fragile - and I found that there's an actual plist containing two pieces of info at /Applications/Xcode.app/Contents/Resources/LicenseInfo. For example, from the Xcode 7.2 beta:

<plist version="1.0">
<dict>
    <key>licenseID</key>
    <string>EA1327</string>
    <key>licenseType</key>
    <string>Beta</string>
</dict>
</plist>

This allows us to more easily grab the agreement number, and know whether it's general release or beta. (licenseType will be either Beta or GM).

Knowing whether it's general release or Beta is important because there are actually two other keys that can be set in com.apple.dt.Xcode:

➜  ~  defaults read /Library/Preferences/com.apple.dt.Xcode
{
    IDELastBetaLicenseAgreedTo = EA1327;
    IDEXcodeVersionForAgreedToBetaLicense = "7.2";
}

Also, if com.apple.dt.Xcode is actually a standard Mac preference, it's better to use defaults to set the prefs rather than writing directly to the plist storage to avoid caching issues with cfprefsd.

Filing this to point out the issue, but I can work on a PR for this.

[timsutton]

Just to note that when I last checked what the DVT framework was doing for handling this, it looked like they're just reading the /L/P/com.apple.dt.Xcode.plist directly, not using Preferences/defaults.