Validate incoming data properly.

Make sure calls to readBuffer() and readEmbeddedBuffer()
get the correct size, parent and offset passed in, so
these can be validated by libhwbinder.

Modified HwBlob to take a length argument as well,
so it can be validated.

Bug: 30498700
Test: hidl_test, hidl_test_java, Youtube, Maps, Netflix, Camera
Change-Id: I28712db97ae29b46acfe952d3d92d1ce5f666a4d