Update default system Proguard config rules

Use the `includedescriptorclasses` modifier with the native keep rules
to ensure native method signatures remain preserved. This matches
common native Proguard rules used elsewhere.

Bug: 196084106
Test: m + presubmit
Change-Id: Ie413459ad0f65ce6b8cd6eb34318494c12568c5e

core/proguard_basic_keeps.flags

@@ -9,7 +9,7 @@
 }
 
 # For native methods, see http://proguard.sourceforge.net/manual/examples.html#native
--keepclasseswithmembernames class * {
+-keepclasseswithmembernames,includedescriptorclasses class * {
     native <methods>;
 }