Use proto for disk usage.
Bug: 79932027
Test: Ran through the following scenarios-
- RebootActivity parses previous boot parameters; sets next as expected.
- Push old parameters to device and ensured bootanimations reads it and
saves new format (serialized proto) that can be read in userspace
(RebootActivity).
- Unit tests pass.
Change-Id: I98aefb7a832c985d5061ee80ab85a4523f067641
81121bd0ac