Fix NPE when trying to clear MWI for invalid subId

VvmPhoneStateListener can be trigger with a when the PhoneAccountHanlde
no longer has a valid subId. why this can happen still needs
investigation.

This CL changed VvmPhoneStateListener to ignore invalid subIds, and
guard PhoneGlobals.clearMwiIndicator against null phone.

Bug: 30315483
Change-Id: I3d062991d8af63aaf0ebc76b3961a335dace3192
2 files changed