Skip dexbootjar check using AllowMissingDependencies flag

Soong checks that every module part of a bootclass_fragment provides a
dex boot jar file, even if the module is not a dependency of vendor.img

Using AllowMissingDepdencies() to skip this check allows vendors to be
more aggressive in removing projects from their source tree

Test: In build/soong, run go test ./...
Test: m nothing
Bug: 196306898
Bug: 192616764
Change-Id: I78b062afdc19a6a3251aa8552230f3fcf334b6fb

java/dexpreopt_bootjars.go

@@ -500,7 +500,11 @@ func copyBootJarsToPredefinedLocations(ctx android.ModuleContext, srcBootDexJars
 		dst := dstBootJarsByModule[name]
 
 		if src == nil {
-			ctx.ModuleErrorf("module %s does not provide a dex boot jar", name)
+			if !ctx.Config().AllowMissingDependencies() {
+				ctx.ModuleErrorf("module %s does not provide a dex boot jar", name)
+			} else {
+				ctx.AddMissingDependencies([]string{name})
+			}
 		} else if dst == nil {
 			ctx.ModuleErrorf("module %s is not part of the boot configuration", name)
 		} else {