Integrate RELEASE_R8_GLOBAL_CHECK_NOT_NULL_FLAGS into soong

Include the new set of `checknotnull.flags` 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: Idf015a3ea5bd9f6e11213f683da008b5706668e0