stats: Set min_sdk_version

Modules contributing mainline modules (APK/APEX) should set
min_sdk_version as well as apex_available.

For now setting min_sdk_version doesn't change build outputs.
But build-time checks will be added soon.

Bug: 152655956
Test: m
Change-Id: Icd184c6fe06a688bd79ef6514bb49dffb706a9a1

libstats/push_compat/Android.bp

@@ -48,6 +48,7 @@ cc_library {
     export_include_dirs: ["include"],
     static_libs: ["libgtest_prod"],
     apex_available: ["com.android.resolv"],
+    min_sdk_version: "29",
 }
 
 cc_test {

libstats/socket/Android.bp

@@ -55,6 +55,7 @@ cc_library_headers {
     export_include_dirs: ["include"],
     host_supported: true,
     apex_available: ["com.android.resolv"],
+    min_sdk_version: "29",
 }
 
 cc_benchmark {