Resolve NPE in post call send message prompt.

This issue occurs when users are shown a post call snackbar and their shared
prefs are updated before pressing "send message". This change resolves the
issue by getting the phone number when the prompt is created rather than when
the button is pressed.

Bug: 62294499
Test: PostCallTest#postCall_UpdatedQuickly_PostCallStartedSuccessfully.
PiperOrigin-RevId: 161689241
Change-Id: Ie2c857f6743aa556f993bc3f8e92c8b2b7357c08