Cache mEphemeralAppsDisabled state

Init mEphemeralAppsDisabled in systemReady when settings provider
is available.

Registered content observers for ENABLE_EPHEMERAL_FEATURE and
WEB_ACTION_ENABLED.

Test: manual triggering enable_ephemeral_feature=1/0
Fixes: 36869938
Fixes: 36895304
Change-Id: I0051c45bb209d86041b967a409f8a95967df21dc