Update libpowermanager to expose enum and export headers.

Move libpowermanager's transaction ID enum (needed when
implementing BnPowerManager::onTransact()) into
IPowerManager.h and update the library's Android.mk file to
export its headers.

Bug: 22122485
Change-Id: Ie97198c3d2ebe8e880125e19a001ef7162009858
diff --git a/services/powermanager/Android.mk b/services/powermanager/Android.mk
index 7b24c65..4deb115 100644
--- a/services/powermanager/Android.mk
+++ b/services/powermanager/Android.mk
@@ -14,4 +14,6 @@
 
 LOCAL_CFLAGS += -Wall -Werror -Wunused -Wunreachable-code
 
+LOCAL_EXPORT_C_INCLUDE_DIRS := $(LOCAL_PATH)/../../include
+
 include $(BUILD_SHARED_LIBRARY)