Minimize window during which default sub may be invalid.

Verified from logs that the window is reduced from ~475ms
to ~28ms.

Test: manually tested single sim<->multi sim transitions
Test: manually verified window duration from logs
Test: atest PhoneConfigurationManagerTest
Bug: 163582235
Change-Id: I75967e2099424997dbb32e4f4c96bd2bf961aac1