Core assumes application will remain valid thru registration process

[iCollin]

Bug Report

It was noticed that Core assumes applications_ entry will be valid throughout the registration process, when the applications_ entry may be removed.

Reproduction Steps

1. Register some apps
2. Change language quickly

Expected Behavior

Registration process completes successfully

Observed Behavior

application == nullptr in either of the following methods: (depending on timing)

commands::RegisterAppInterfaceRequest::CheckLanguage()
sdl_rpc_plugin::commands::FinishSendingResponseToMobile(...)

OS & Version Information

• OS/Version: ubuntu 18.04
• SDL Core Version: develop
• Testing Against: Add test for 3805 crash during language change and registration sdl_atf_test_scripts#2602

[iCollin]

Closed via #3815