Stop disabling Metalava quality checks

These checks have been disabled since the initial integration of
metalava in the build system. Stop disabling them, but baseline existing
issues.

For the checks that are disabled for good reason, leave a comment that
explains why.

Bug: 306806877
Test: m checkapi
Change-Id: Ica5c28f798b4aee0ae3722a0bd176bcc621d994e