Introduce Intent#prepareToEnterSystemServer()

Put all operations that need to be performed to an intent upon
receiving it in system_server into its own dedicated method for easier
maintenance in the future.

- Prevent leaked file descriptors
- Remove all system only flags and extended flags

Bug: 326100473
Test: TH
Flag: EXEMPT refactor
Change-Id: Id2931d1c1d8b8f1caf4a92e4e2357c1f2e9d0925