Create RO flag for BAL strict mode

Create a read only flag to fix issues which lead to having to roll back
the strict mode feature implementation.
Background: strict mode (esp. the detectAll method) might be used during
startup, before the whole flag infrastructure is initialized.

Flag: com.android.window.flags.bal_strict_mode_ro
Bug: 324089586
Test: atest BackgroundActivityLaunchTest
Change-Id: I0100b0cd20592a2711026c6648eaec956ff4ca5c