Generate info about the selected apex variant in platform builds

out/soong/prebuilt_info.json will contain information about whether
source or prebuilt of an apex was used. If prebuilt is used, it will
print the path to its prebuilt_info file. This file will be
used to pick a matching set of MTS test prebuilts.

Each apex will have an entry in this file with the following properties
- Name, mandatory
- Is_prebuilt, mandatory
- Prebuilt_info_file_path, optional

Implementation details
- Introduce a new `prebuilt_info` prop to `apex_set` and `prebuilt_apex`
- All apex module types will set a prebuiltInfoProvider in
  GenerateAndroidBuildActions, including the source apex module types
- Create a `apex_prebuiltinfo_singleton` that visits all apex modules.
  It uses `IsHideFromMake` to filter out the unselected variants of a
  specific apex. This new singleton will create prebuilt_info.json
- Dist prebuilt_info.json for droidcore

Test: m nothing --no-skip-soong-tests
Test: m out/soong/prebuilt_info.json
Test: ls -l out/soong/prebuilt_info.json --human-readable
-rw------- 1 spandandas primarygroup 25K Mar 11 23:46 out/soong/prebuilt_info.json

Test: #modified trunk_staging.locally to select prebuilts of some
mainline modules. Spot-checked that `is_prebuilt` and
`prebuilt_info_file_path` get populated appropriately

Bug: 327480225

Change-Id: I65c73010142b034ad1d2b3d05ef493be034e8d74

apex/apex.go

@@ -2424,6 +2424,33 @@ func (a *apexBundle) GenerateAndroidBuildActions(ctx android.ModuleContext) {
 
 	// Set a provider for dexpreopt of bootjars
 	a.provideApexExportsInfo(ctx)
+
+	a.providePrebuiltInfo(ctx)
+}
+
+var prebuiltInfoProvider = blueprint.NewProvider[prebuiltInfo]()
+
+// contents of prebuilt_info.json
+type prebuiltInfo struct {
+	// Name of the apex, without the prebuilt_ prefix
+	Name string
+
+	Is_prebuilt bool
+
+	// This is relative to root of the workspace.
+	// In case of mainline modules, this file contains the build_id that was used
+	// to generate the mainline module prebuilt.
+	Prebuilt_info_file_path string `json:",omitempty"`
+}
+
+// Set prebuiltInfoProvider. This will be used by `apex_prebuiltinfo_singleton` to print out a metadata file
+// with information about whether source or prebuilt of an apex was used during the build.
+func (a *apexBundle) providePrebuiltInfo(ctx android.ModuleContext) {
+	info := prebuiltInfo{
+		Name:        a.Name(),
+		Is_prebuilt: false,
+	}
+	android.SetProvider(ctx, prebuiltInfoProvider, info)
 }
 
 // Set a provider containing information about the jars and .prof provided by the apex