Use allowBalExemptionForSystemProcess

Instead of the existing BackgroundActivityStartPrivileges typed
`forcedBalByPiSender` use a simple boolean
`allowBalExemptionForSystemProcess` to allow the sender to use the
guarded exemption (start if system process) when allowing BAL.

The idea is to completely replace this with an upgrade of
`MODE_BACKGROUND_ACTIVITY_START_ALLOWED` to
`MODE_BACKGROUND_ACTIVITY_START_ALLOW_ALWAYS`.

Test: atest WmTests CtsWindowManagerBackgroundActivityTestCases
Flag: EXEMPT refactoring
Bug: 374988683
Change-Id: I4846796722dadcf0c212acf8b547fe6a1da7f676