Implement directed vendor snapshot

Vendors can now generate only needed modules by setting the following
Makefile variables:

- DIRECTED_VENDOR_SNAPSHOT: set to true
- VENDOR_SNAPSHOT_MODULES: list of snapshot candidates

e.g.

DIRECTED_VENDOR_SNAPSHOT := true
VENDOR_SNAPSHOT_MODULES := toybox_vendor sh_vendor libbase libcutils ...

Bug: 157967325
Test: m dist vendor-snapshot after setting those in BoardConfig.mk
Change-Id: I6515a43d9358d94483d7c7fa2b066f9dd457f6aa

android/config.go

@@ -1401,6 +1401,14 @@ func (c *deviceConfig) BoardReqdMaskPolicy() []string {
 	return c.config.productVariables.BoardReqdMaskPolicy
 }
 
+func (c *deviceConfig) DirectedVendorSnapshot() bool {
+	return c.config.productVariables.DirectedVendorSnapshot
+}
+
+func (c *deviceConfig) VendorSnapshotModules() map[string]bool {
+	return c.config.productVariables.VendorSnapshotModules
+}
+
 // The ConfiguredJarList struct provides methods for handling a list of (apex, jar) pairs.
 // Such lists are used in the build system for things like bootclasspath jars or system server jars.
 // The apex part is either an apex name, or a special names "platform" or "system_ext". Jar is a