retire Android.mk to prevent AOSP from complaining

we don't use Android.mk for AOSP, but we do for the NDK JNI build. Let's
rename it so that the latest AOSP 15 QPR 1 stops complaining.

libs/lpac-jni/build.gradle.kts

@@ -32,7 +32,7 @@ android {
     }
     externalNativeBuild {
         ndkBuild {
-            path("src/main/jni/Android.mk")
+            path("src/main/jni/lpac-jni.mk")
         }
     }
     compileOptions {