Add global rule for -keepattributes Exceptions

R8 full mode requires an explicit keep rule for keeping attributes for a
given target (class/method/member). As such, we can set the global rule
for keeping these attributes with minimal cost while reducing the
maintenance burden for adding incremental keep rules.

For a typical Android build, this adds <20KB total to the image.

Bug: 215530220
Test: m
Change-Id: Iad3de32e27da0ca93b618d8a203ee65bae0bb5d1

core/proguard_basic_keeps.flags

@@ -10,6 +10,12 @@
 # and RuntimeVisibleTypeAnnotations, as well as associated defaults.
 -keepattributes RuntimeVisible*Annotation*,AnnotationDefault
 
+# With R8 full mode, certain attributes are only kept when matched with an
+# explicit keep rule for that target, even with a global -keepattributes rule.
+# As such, we can add the global keep rule here with minimal cost while
+# simplifying incremental development.
+-keepattributes Exceptions
+
 # For enumeration classes, see http://proguard.sourceforge.net/manual/examples.html#enumerations
 -keepclassmembers enum * {
     public static **[] values();