Generate info about the selected app variant in platform builds

This is a followup to aosp/2999198 and adds information about apps.

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

Implementation details
- Move prebuiltInfoProvider out of build/soong/apex to
  build/soong/android. This allows build/soong/java to use it.
- Introduce a new `prebuilt_info` prop to `android_app_set` and
  `android_app_import`
- All app module types will set a prebuiltInfoProvider in
  GenerateAndroidBuildActions, including the source app module types

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 317K Mar 11 23:46 out/soong/prebuilt_info.json

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

Bug: 327480225
Change-Id: I5078e0ec26c9568194550909962b90111a5223f7

android/apex.go

@@ -976,3 +976,18 @@ type ApexExportsInfo struct {
 	// Map from the apex library name (without prebuilt_ prefix) to the dex file path on host
 	LibraryNameToDexJarPathOnHost map[string]Path
 }
+
+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"`
+}