vendor/lineage: Add support for java source overlays

*) Set JAVA_SOURCE_OVERLAYS in a device makefile as follows:
   # Java sourcefile overlays (for Android.bp built modules only)
   # Format is a whitespace separated set of rules, each of which
   # structured as follows:
   # modulename|overlay directory|glob pattern within overlay dir
   JAVA_SOURCE_OVERLAYS := \
           org.lineageos.hardware|$(LOCAL_PATH)/lineagehw|**/*.java

*) As of this commit, java sources overlays are only allowed for
   module org.lineageos.hardware.

Change-Id: I6be1c12567081357f5231a84df98ac002c0563b4

build/soong/android/variable.go

@@ -30,6 +30,7 @@ type Product_variables struct {
 type ProductVariables struct {
 	Additional_gralloc_10_usage_bits  *string `json:",omitempty"`
 	Has_legacy_camera_hal1  *bool `json:",omitempty"`
+	Java_Source_Overlays *string `json:",omitempty"`
 	Needs_text_relocations  *bool `json:",omitempty"`
 	Specific_camera_parameter_library  *string `json:",omitempty"`
 	Target_shim_libs  *string `json:",omitempty"`