Extract proguard rules found in imported JAR archives

In Android Studio R8 is able to use bundled proguard
rules found both in AAR archives (under /proguard.txt)
and JAR archives (under /META-INF/proguard/*)

Soong currently only handles the _easier_ AAR format
and ignores the possibility to have rules in JAR archives.

To support those rules we use the newly introduced
ExtractR8Rules cli tool in R8 to extract all the embedded
rules in the imported JAR for later propagation.

Bug: 377144587
Test: m, including okhttp-4.12.0.jar now doesn't create R8 warnings.
Change-Id: I10f9bbf76bc8daec1ec470fb493e399d1beffc8a
diff --git a/java/config/config.go b/java/config/config.go
index 19d6952..2ed2953 100644
--- a/java/config/config.go
+++ b/java/config/config.go
@@ -174,6 +174,7 @@
 	pctx.HostBinToolVariable("ApiCheckCmd", "apicheck")
 	pctx.HostBinToolVariable("D8Cmd", "d8")
 	pctx.HostBinToolVariable("R8Cmd", "r8")
+	pctx.HostBinToolVariable("ExtractR8RulesCmd", "extract-r8-rules")
 	pctx.HostBinToolVariable("ResourceShrinkerCmd", "resourceshrinker")
 	pctx.HostBinToolVariable("HiddenAPICmd", "hiddenapi")
 	pctx.HostBinToolVariable("ExtractApksCmd", "extract_apks")