libgui: Make BitTube Parcelable and use unique_fd

This change completes the Parcelable interface for BitTube (it was
semi-Parcelable before as it implemented writeToParcel, but this adds
the complementary readFromParcel).

It also changes the send and receive file descriptors from ints to
android::base::unique_fds, which simplifies some of their lifecycle
management.

Finally, it changes the default constructor to leave the class
uninitialized, adding a BitTube(BitTube::DefaultSize) constructor to
replace that functionality (and paving the way for the ability to
default-construct a BitTube prior to readFromParcel'ing into it).

Test: m -j + manual testing
Change-Id: Ib0cba4c7c443b449a9a1837f07f7334395d4f10d