Add global checknotnull.flags Proguard config

This config is conditionally used based on the build flag,
allowing `-convertchecknotnull` R8 optimizations for a class of common
null checks.

Bug: 280633711
Test: build-flag set RELEASE_R8_GLOBAL_CHECK_NOT_NULL_FLAGS && m
Flag: RELEASE_R8_GLOBAL_CHECK_NOT_NULL_FLAGS
Change-Id: If17e7fc2c5b3306f4abaa28f310b3260c6d6294a