Fixed incremental iOS builds broken by f759cd3

Improved Podfile parsing when performing incremental builds.
This is still not really correct Podfile parsing as it doesn't
handle escaped characters within each property value of a "pod"
line.

This also adds support for all currently supported Podfile pod
declaration properties when adding Cocoapod references via XML and
fixes warnings when adding identical pod references when appending
to builds with an existing Podfile.

Bug: 122456935
Issue googlesamples#176

Change-Id: If502bc118c4d531484a81e4ec4e888874ded91be

Author: Stewart Miles

sample/Assets/PlayServicesResolver/Editor/SampleDependencies.xml

@@ -41,14 +41,28 @@
            Name of the Cocoapod.
          * "path" (optional)
            Path to the local Cocoapod.
+           NOTE: This is expanded to a local path when the Podfile is generated.
+           For example, if a Unity project has the root path "/my/game" and the
+           pod the path is "foo/bar", this will be will be expanded to
+           "/my/game/foo/bar" when the Podfile is generated.
          * "version" (optional)
            Cocoapod version specification for the named pod.
            If this is not specified the latest version is used.
+           NOTE: This can't be used when "path" is set.
          * "bitcodeEnabled" (optional)
            Whether this Cocoapod requires bitcode to be enabled in Unity's
            generated Xcode project.  This is "true" by default.
          * "minTargetSdk" (optional)
-           The minimum iOS SDK required by this Cocoapod. -->
+           The minimum iOS SDK required by this Cocoapod.
+         * "configurations" (optional)
+           Podfile formatted list of configurations to include this pod in.
+         * "modular_headers" (optional)
+           Set to true to enable modular headers, false to disable.
+         * "source" (optional)
+           Source repo to fetch the pod from.
+         * "subspecs" (optional)
+           Subspecs to include for the pod.
+     -->
     <iosPod name="Google-Mobile-Ads-SDK" path="../google-mobile-ads-sdk" version="~> 7.0" bitcodeEnabled="true"
             minTargetSdk="6.0">
       <!-- Set of source URIs to search for this Cocoapod spec.