Always return STANDARD & CUSTOM game modes as available modes

Change the default game mode from UNSUPPORTED to STANDARD when config
is missing etc. This will solve the ambiguity on UNSUPPORTED as it's
publicly documented to always return from getGameMode when application
is not a game instead.

Bug: b/243448953
Test: atest GameManagerServiceTests
Change-Id: I277735cccd0daf6148aa106cc31bf92f73db018a