Improve buffer ownership for PartitionBlockDevice

PartitionBlockDevice currently only accepts borrowed slice &mut [u8] as
buffers. This can be awkward as user needs to separately maintain a
scratch buffer and GPT buffer for each instance, resulting in serveral
intermediate data structures needed. The CL changes the buffer type into
a generic type that implements `DerefMut<Target = [u8]>` to make the
choice of ownership model more flexible for the user.

The main changes are definitions of GblDisk (renamed from
PartitionBlockDevice) and the return type of the `GblOps::disk()`
interface (renamed from GblOps::partitions()). The rest of changes are
simply name replacement.

Bug: 376693338

Change-Id: I818526b33d574acf2dea6687b4a564e9f916ff77