Keep proguard fields for lite runtime

The lite proto runtime uses reflection to access fields based on the names in
the schema, keep all the fields.

Bug: 117607748
Bug: 140062984
Test: m checkbuild
Change-Id: I39bc7d28f619dd2ccaee603e4d407aa7ad33f534
Merged-In: I39bc7d28f619dd2ccaee603e4d407aa7ad33f534

core/proguard_basic_keeps.flags

@@ -72,3 +72,7 @@
 
 # Less spammy.
 -dontnote
+
+# The lite proto runtime uses reflection to access fields based on the names in
+# the schema, keep all the fields.
+-keepclassmembers class * extends com.google.protobuf.MessageLite { <fields>; }