Allow apps receiving onMediaButton() callback to start FGS from BG.

Bug: 174699413
Test: manual - verified that the app receiving onMediaButton()
      callback is able to start FGS only when the caller is in a
      state to start FGS.
CTS-Coverage-Bug: 174699413
Change-Id: I2dea43b79aa48fde83c6c3da2ad1454b29d60867