Enable Explicit API mode and sysui lint for pods

Modify java_defaults for sysui pods to add -Xexplicit-api=warning.
(a follow-up CL will change this to strict). This will help ensure that
no declarations are unintentionally exposed to the public API of pods,
which would negatively affect Soong build times.

Also, enable sysui custom lint checks for pods.

Test: m, flashall
Bug: 307607958
Flag: EXEMPT build change is a no-op
Change-Id: Ib01bfd12f1773f3db4e9fde0ec2798c1b5912ad5