[MS83.1] Make some APIs module-lib instead of system current

Some of the internal classes are not available for priv-apps.
Thus, make them module-lib instead.

This change also adds some nullability annotations for existing
APIs according to go/android-api-guidelines.

Test: m
Test: m frameworks-base-api-system-current-compat
Fix: 217479745
Change-Id: I18de46b11df7232ab82b9465856fdde621283156