Remove memory leak in PersistentDataBlockService jni

When calling the methods
 com_android_server_PersistentDataBlockService_getBlockDeviceSize()
 com_android_server_PersistentDataBlockService_wipe()
memory is leaked because string created by
 GetStringUTFChars() is not released.

Use ScopedUtfChars instead to ensure that memory is released.

Change-Id: I880a6d66a4824778b411b858774b8ffa009c1e17