health 1.0: add helper library for -impl

Now that libhealthd is no longer recognized as a HAL_STATIC_LIBRARY,
vendors must define their own android.hardware.health@1.0-impl module.
Add a helper and README.md for them to explain this transition.

Test: builds
Bug: 127677771
Change-Id: I65c162e7b5caed93c39a4a1cb6a2893fbb25724b
diff --git a/health/1.0/default/Android.bp b/health/1.0/default/Android.bp
index e4e6ce7..7581335 100644
--- a/health/1.0/default/Android.bp
+++ b/health/1.0/default/Android.bp
@@ -18,10 +18,9 @@
 
 }
 
-cc_library_shared {
-    name: "android.hardware.health@1.0-impl",
+cc_library_static {
+    name: "android.hardware.health@1.0-impl-helper",
     vendor: true,
-    relative_install_path: "hw",
     srcs: ["Health.cpp"],
 
     header_libs: [
@@ -39,6 +38,17 @@
 
     static_libs: [
         "android.hardware.health@1.0-convert",
+    ],
+}
+
+cc_library_shared {
+    name: "android.hardware.health@1.0-impl",
+    vendor: true,
+    relative_install_path: "hw",
+
+    static_libs: [
+        "android.hardware.health@1.0-impl-helper",
+        "android.hardware.health@1.0-convert",
         "libhealthd.default",
     ],
 }