[PostBoot] Fix bug where post boot shader has a flicker

Pass correct default value -1 when getting RUNTIME_STATE_PENDING_REQUEST_CODE when it is not set.

This will avoid accidentally starting ProxyActivityStarter from Launcher#onCreate(), which will cause a flicker for post boot animation

Fix: 375308654
Flag: NONE - xml flag not supported
Test: manual
Change-Id: Ic3888508657b5b7aab37f6022768d53007bf9c00