Add COLUMN_CAPACITY_BYTES column to DocumentsContract.

Alternatives were considered:
- COLUMN_CAPACITY dounds more like real english sentence, but it is
  inconsistent with consistent with COLUMN_AVAILABLE_BYTES.
- COLUMN_USED_BYTES is consistent with COLUMN_AVAILABLE_BYTES but we rarely
  care about used bytes, more about total capacity. Such information is also
  rarely exposed by backends, and would have to be computed, so misleading.

BUG=None

Change-Id: Ic1ff3eb8fc9ec12cc79ee78c490f361f77cb4706