Add proto.canonical_path_from_root

Historically, we've always passed '-I .' as the first argument to
protoc, essentially treating all proto file package names as their full
path in the android source tree. This would make sense in a monorepo
world, but it makes less sense when we're pulling in external projects
with established package names.

So keep the same default (for now), but allow individual builds to opt
into using local paths as the default names with
'canonical_path_from_root: false'. A cleanup effort and/or large scale
change in the future could change the default to false.

As part of this, run protoc once per input proto file, since the flags
may need to change per-file. We'll also need this in order to specify
--dependency_out in the future.

Bug: 70704330
Test: aosp/master build-aosp_arm.ninja is identical
Test: aosp/master soong/build.ninja has expected changes
Test: m
Test: Build protobuf test
Change-Id: I9d6de9fd630326bbcced1c62a4a7e9546429b0ce

java/builder.go

@@ -169,6 +169,7 @@ type javaBuilderFlags struct {
 	protoFlags       []string
 	protoOutTypeFlag string // The flag itself: --java_out
 	protoOutParams   string // Parameters to that flag: --java_out=$protoOutParams:$outDir
+	protoRoot        bool
 }
 
 func TransformKotlinToClasses(ctx android.ModuleContext, outputFile android.WritablePath,