API: Clean up redundant and ineffective usages of SystemApi and TestApi

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
Change-Id: Ibd5d6a22862fdbc1e20a1cb3925280f5a682edea
Test: METALAVA_PREPEND_ARGS="--error UnhiddenSystemApi" m checkapi
Exempt-From-Owner-Approval: API cleanup