Bug 1091087 - Disable MOZ_NATIVE_DEVICES when MOZ_ANDROID_RESOURCE_CONSTRAINED is set. r=glandium

This saves dexing and shipping the Google Play Services and other Google
libraries, which add resources and about 3megs of code.

Due to ordering issues, the relevant flags and toggles were moved to
configure.in and exposed early enough to be used by confvars.sh.
Bug 976699 - Place architecture specific assets in assets/$(ANDROID_CPU_ARCH). r=glandium