Introduce BOARD_CURRENT_API_LEVEL_FOR_VENDOR_MODULES

If BOARD_CURRENT_API_LEVEL_FOR_VENDOR_MODULES has a numeric value,
it replaces "current" or "system_current" with the version which
the flag indicates.

Bug: 163009188
Test: BOARD_CURRENT_API_LEVEL_FOR_VENDOR_MODULES=29 m, and then check if every vendor
java module's sdk_version is 29 if its sdk_version was current.

Change-Id: I17b49b8e02caf2d1bc57b91648d4420f3ad9fcb9
diff --git a/android/config.go b/android/config.go
index 13e55bc..dd622e5 100644
--- a/android/config.go
+++ b/android/config.go
@@ -35,6 +35,7 @@
 
 var Bool = proptools.Bool
 var String = proptools.String
+var StringDefault = proptools.StringDefault
 
 const FutureApiLevel = 10000
 
@@ -958,6 +959,10 @@
 	return String(c.config.productVariables.DeviceVndkVersion)
 }
 
+func (c *deviceConfig) CurrentApiLevelForVendorModules() string {
+	return StringDefault(c.config.productVariables.DeviceCurrentApiLevelForVendorModules, "current")
+}
+
 func (c *deviceConfig) PlatformVndkVersion() string {
 	return String(c.config.productVariables.Platform_vndk_version)
 }