Fix flattening/unflattening of android::Sensor.

Write string lengths as uint32_t so that their width is
the same on 32 and 64 bit processes.

Note that this fixes another bug as a side effect; getFlattenedSize
was assuming that sizeof(uint32_t) == sizeof(size_t).

Change-Id: I7b6e3993e1f1ac45c14832ce59c59e0772855a2f