Fix deadlock between PMS and UMS via writeSettings

Move getting active users out of mLock to fix deadlock.

Symptom:
PMS: hold mLock      ->  UMS: ask mUsersLock
UMS: hold mUsersLock ->  PMS: ask mLock

Bug: 407169095
Flag: EXEMPT bugfix
Test: atest PackageManagerServiceServerTests
Test: atest CtsPackageInstallTestCases
Change-Id: I7d0978eb32f1e15d2aff66aca6332ec573d26212