API: Clean up redundant and ineffective usages of SystemApi and TestApi (2/2)

Everything that is marked SystemApi or TestApi, but not @hide is still
part of the public SDK, it is therefore not sound to have that combination.

In the future, specifing such a combination will be considered an error
to prevent inadvertently exposing SystemApi and TestApi as public API.

Bug: 115333477
Test: METALAVA_PREPEND_ARGS="--error UnhiddenSystemApi" m checkapi
Change-Id: I6d370e8ffcb9e7cd2859c310c555a2a847c70ae8
Exempt-From-Owner-Approval: API cleanup