[installd] Modernize codebase by replacing NULL with nullptr

Fixes -Wzero-as-null-pointer-constant warning.

Test: m
Bug: 68236239

(cherry picked from commit e7bf377d62141c27508b28513e2091a826ad7b15)

Merged-In: I9dc0c1020993ec3c7e95ab552f7620ea2badf742
Change-Id: I1937de4d243e213c36f6f9c3e67380f019c6e61c
6 files changed