[REFACTOR] DynamicPartitionControl: minimize API

- Move UpdatePartitionMetadata in boot_control_android.cc to
  DynamicPartitionControlAndroid::PreparePartitionsForUpdate to
  reflect code ownership better.
- Minimize the API by removing unused functions now that we have
  PreparePartitionsForUpdate
- Fix tests: replace MockDynamicPartitionControl to
  MockDynamicPartitionControlAndroid because it tests
  DynamicPartitionControlAndroid.

Test: unit test
Test: manually apply full OTA
Change-Id: I2959270bd89c4f8ddaa45cf45ba93acdec850f67
diff --git a/mock_dynamic_partition_control.h b/mock_dynamic_partition_control.h
index cdfeecc..310e528 100644
--- a/mock_dynamic_partition_control.h
+++ b/mock_dynamic_partition_control.h
@@ -21,11 +21,14 @@
 
 #include <gmock/gmock.h>
 
+#include "update_engine/common/boot_control_interface.h"
+#include "update_engine/dynamic_partition_control_android.h"
 #include "update_engine/dynamic_partition_control_interface.h"
 
 namespace chromeos_update_engine {
 
-class MockDynamicPartitionControl : public DynamicPartitionControlInterface {
+class MockDynamicPartitionControlAndroid
+    : public DynamicPartitionControlAndroid {
  public:
   MOCK_METHOD5(MapPartitionOnDeviceMapper,
                bool(const std::string&,