Move the android_platform next to it's entrypoint product config file

Because we're going to start generating partition images for the
product, and the partitions will eventually be checked in, we want
them to be in sensible locations. And the platform should be there as
well so all the targets for a product are co-located.

Bug: 297269187
Test: m nothing && b build --config=android //build/bazel/examples/apex/minimal:build.bazel.examples.apex.minimal
Change-Id: Iaa25c44aa00295ada279d5fd49b5498bbafb89d5

android/variable.go

@@ -484,6 +484,12 @@ type ProductVariables struct {
 	KeepVndk *bool `json:",omitempty"`
 
 	CheckVendorSeappViolations *bool `json:",omitempty"`
+
+	// PartitionsVars are extra variables that are used to define the partition images. They should
+	// not be read from soong modules.
+	PartitionVars struct {
+		ProductDirectory string `json:",omitempty"`
+	} `json:",omitempty"`
 }
 
 func boolPtr(v bool) *bool {