Update signature files to the new format

The new format is described in tools/metalava/FORMAT.md.

Fixes: 116589402 Switch signature files over to the v2 format
Fixes: 112626813 Drop "synchronized" modifier from the signature format
Fixes: 122358225 Omit overriding methods that only differ in final [...]
Fixes: 73088715 API Review: Need doclava to output nullability
Fixes: 79739773 API Review: Metalava enforcing constants are constant
Exempt-From-Owner-Approval: Large-scale tooling change
Test: make sdk, make update-api, make checkapi
Change-Id: I8314f4e7099fa92e4f8ed7d283ccf836cc9a84a0
Merged-In: Ia248aece5250e84e47c815c601133b698bf644c2