Add apex variants

Now, APEX-aware modules (the ones implementing android.ApexModule
interface) are created with multiple variants for each APEX that they
are included.

For example, if a module is included (either directly or indirectly -
via static linking) to two APEXs, the module is built separately for the
two APEXs (and of course separately for platform). This is a first step
to limit the symbol visibility to the modules built for APEXs; platform
private symbols and libs shouldn't be allowed for them.

In addition, the build system now tracks transitive dependencies of
the modules in APEXs. For example, if
native_shared_lib_modules:["libFoo"] then libFoo and its dependencies
are all automatically included to the APEX.

Bug: 112672359
Test: m apex.test; the built apex has additional libs (such as liblog,
libc++, ...) that are dependencies of the ones specified in Android.bp

Change-Id: Id9e3fc486dd4e7e36f8b6799dfb041868c5198d5

Android.bp

@@ -336,6 +336,22 @@ bootstrap_go_package {
     pluginFor: ["soong_build"],
 }
 
+bootstrap_go_package {
+    name: "soong-apex",
+    pkgPath: "android/soong/apex",
+    deps: [
+        "blueprint",
+        "soong",
+        "soong-android",
+        "soong-cc",
+        "soong-java",
+    ],
+    srcs: [
+        "apex/apex.go",
+    ],
+    pluginFor: ["soong_build"],
+}
+
 //
 // Defaults to enable various configurations of host bionic
 //