Remove POD helper classes for BitTableBuilder.

Instead of declaring the classes explicitly and then casting,
create generic BitTableBuilder::Entry class for that purpose.
This removes the need to keep the POD helper classes in sync.

Test: test-art-host-gtest-stack_map_test
Test: test-art-host-gtest-bit_table_test
Change-Id: I4c632313bafd3a4bc823648436a5310b6f2a1d13