android_app_import is now ApexModule

Like other prebuilts (java_import, etc.), android_app_import has now
became ApexModule. It means it supports APEX-specific properties like
apex_available, min_sdk_version, etc.

Bug: 154488897
Test: m
Change-Id: I86f02771496ffc68c98a50ddfc786788fa1e05fb

apex/apex_test.go

@@ -3428,6 +3428,7 @@ func TestApexWithAppImports(t *testing.T) {
 			dex_preopt: {
 				enabled: false,
 			},
+			apex_available: ["myapex"],
 		}
 
 		android_app_import {
@@ -3439,6 +3440,7 @@ func TestApexWithAppImports(t *testing.T) {
 				enabled: false,
 			},
 			filename: "AwesomePrebuiltAppFooPriv.apk",
+			apex_available: ["myapex"],
 		}
 	`)
 
@@ -3480,6 +3482,7 @@ func TestApexWithAppImportsPrefer(t *testing.T) {
 			filename: "AppFooPrebuilt.apk",
 			presigned: true,
 			prefer: true,
+			apex_available: ["myapex"],
 		}
 	`, withFiles(map[string][]byte{
 		"AppFooPrebuilt.apk": nil,