Set min_sdk_version to be part of mainline modules

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: I2f739d776e40d4eb28fd29d3cd737b81ad2dd75a
diff --git a/Android.bp b/Android.bp
index ddfc29d..1b0e2e7 100644
--- a/Android.bp
+++ b/Android.bp
@@ -8,6 +8,7 @@
             enabled: false,
         },
     },
+    min_sdk_version: "29",
 }
 
 cc_library_headers {
@@ -20,6 +21,7 @@
             enabled: false,
         },
     },
+    min_sdk_version: "29",
 }
 
 subdirs = ["libFLAC"]