MappedByteBuffer get(byte[]) and get(byte[], int, int) weren't updating 'position'.

The majority of this change is work to extend our tests to cover
MappedByteBuffer too, plus new tests to check that 'position' is
always updated correctly by ByteBuffer's relative get/put methods.

Bug: 3291927
Change-Id: I58217de45ba65675730f30771fba592cb2af28ff
tree: 4b825dc642cb6eb9a060e54bf8d69288fbee4904

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