Add sdk mutator for native modules

Compiling native modules against the NDK disables platform features
like ASAN.  For anything shipped on the system image there is no
reason to compile against the NDK.  Add a new mutator to Soong that
creates a platform and an SDK variant for modules that set
sdk_version, and ignore sdk_version for the platform variant.  The
SDK variant will be used for embedding in APKs that may be installed
on older platforms.  Apexes use their own variants that enforce
backwards compatibility.

Test: sdk_test.go
Test: TestJNIPackaging
Bug: 149591340
Change-Id: I7d72934aaee2e1326cc0ba5f29f51f14feec4521

cc/linker.go

@@ -158,6 +158,13 @@ type BaseLinkerProperties struct {
 			// the ramdisk variant of the C/C++ module.
 			Exclude_static_libs []string
 		}
+		Platform struct {
+			// list of shared libs that should be use to build the platform variant
+			// of a module that sets sdk_version.  This should rarely be necessary,
+			// in most cases the same libraries are available for the SDK and platform
+			// variants.
+			Shared_libs []string
+		}
 	}
 
 	// make android::build:GetBuildNumber() available containing the build ID.
@@ -255,6 +262,10 @@ func (linker *baseLinker) linkerDeps(ctx DepsContext, deps Deps) Deps {
 		deps.WholeStaticLibs = removeListFromList(deps.WholeStaticLibs, linker.Properties.Target.Recovery.Exclude_static_libs)
 	}
 
+	if !ctx.useSdk() {
+		deps.SharedLibs = append(deps.SharedLibs, linker.Properties.Target.Platform.Shared_libs...)
+	}
+
 	if ctx.toolchain().Bionic() {
 		// libclang_rt.builtins and libatomic have to be last on the command line
 		if !Bool(linker.Properties.No_libcrt) {