Always allow startForeground(SHORT_SERVICE) on running short FGS

- Previously, call to `startForeground(SHORT_SERVICE)` to an already
running short FGS was only allowed when the app was in a BFSL-allowed
situation.

- Now, this call is always allowed. However, we extend the short FGS
timeout, only when the app is in a BFSL-allowed situation.

Fix: 269369028
Bug: 257270313
Test: Manually tested with a test app, but with the ACTIVITY_STARTER
  BFSL check disabled. It seems like ToT udc-dev has a bug where it's
  triggered for BG activities too.
Test: atest CtsShortFgsTestCases
Change-Id: Ife0527236ab5ccbc55c7a7759251fa4a16664297