I don't see why you think WP7 is a competitor to Google but not Apple. Philosophy-wise, it is somewhere between the iOS and Android platforms. More locked down than Android, less locked down than iOS. Like Android, WP7 is currently distributed by OEMs, independently from Microsoft. But the licensing agreement for those OEMs is much tighter than it is for Android. Like iOS, Microsoft has an established entertainment distribution channel centered around XBox Live and the Zune technology. Even if they think Android is an easier target, I guarantee you they consider Apple a competitor.
In any case, I don't see why you think OEMs are particularly dependent on Google. Google has no direct control over Android -- it's just that Google's fork of the Android source code is the most popular with OEMs for the time being. There is nothing stopping OEMs from building their own CyanogenMod-type customizations (after all, Samsung just hired the creator of this ROM) or even forking Android entirely as apparently some Chinese OEMs are doing: http://arstechnica.com/gadgets/news/2010/07/androids-ascent-.... Hell, there's nothing stopping Microsoft OEMs from releasing their own customization of Android with deep integration with XBox Live, Bing search, and Microsoft's other cloud services.