Add AbstractBlock parent class for Block

Add AbstractBlock parent class for Block

This anticipates I83bceb5e5, which refactors Block into different
classes: Block (for blocks stored in the database), SystemBlock
(for temporary blocks), and AbstractBlock (the parent class).

Block should be become a deprecated alias of DatabaseBlock.

This adds an empty AbstractBlock parent class, and makes Block
extend AbstractBlock, but leaves it otherwise unchanged. This is
to allow typehints to be updated, to avoid a breaking change.

Bug: T222737
Change-Id: I3cf78cf77ccf492dadf53e479f81891961021469