Monitor changes to INSTANT_APPS_ENABLED for all users.

PackageManagerService listens for changes to Secure.INSTANT_APPS_ENABLED
for USER_SYSTEM. Before this change, if instant_apps_enabled is set to
0 for a secondary user, packagemanager would continue to allow ephemeral
apps for the secondary user.

With this change, instant_apps_enabled can be set on a per-user basis.

bug: 121169664
Test: adb shell am switch-user 10
adb shell settings put --user 10 secure instant_apps_enabled 1
cts-tradefed run cts-dev -a arm64-v8a -m CtsAppSecurityHostTestCases -t android.appsecurity.cts.EphemeralTest#testNormalStartEphemeral

Change-Id: Id268550af2a48a6c5a045cdd6ae5e94d195d6087