Move SDK system-image source.prop_templates in their own folder.

This allows to have a different rule to generate the source.properties
based on the templates and place them in a directory that depends on
the current target CPU ABI. In turn this prevents the build from
mixing different source.props when building different targets.

Change-Id: I4a1d9f47426c94f376947ddae8be9f5549331616

sys-img/images_x86_64_source.prop_template

@@ -0,0 +1,9 @@
+Pkg.Desc=Android SDK Platform ${PLATFORM_VERSION}
+Pkg.UserSrc=false
+Pkg.Revision=1
+AndroidVersion.ApiLevel=${PLATFORM_SDK_VERSION}
+AndroidVersion.CodeName=${PLATFORM_VERSION_CODENAME}
+SystemImage.Abi=${TARGET_CPU_ABI}
+SystemImage.GpuSupport=true
+SystemImage.TagId=default
+