Add DeviceConfig and OncePer objects

Add DeviceConfig to store per-device configuration information.  Put a
OncePer object inside Config and DeviceConfig, which computes a value
once per key per object to allow build logic to store arbitrary
per-build or per-device computed values.

Change-Id: I1a38b426f29d223ef5e803e0d4d9604500de2fd2

android/module.go

@@ -56,6 +56,7 @@ type androidBaseContext interface {
 	Darwin() bool
 	Debug() bool
 	AConfig() Config
+	DeviceConfig() DeviceConfig
 }
 
 type BaseContext interface {
@@ -535,6 +536,10 @@ func (a *androidBaseContextImpl) AConfig() Config {
 	return a.config
 }
 
+func (a *androidBaseContextImpl) DeviceConfig() DeviceConfig {
+	return DeviceConfig{a.config.deviceConfig}
+}
+
 func (a *androidModuleContext) Proprietary() bool {
 	return a.module.base().commonProperties.Proprietary
 }