Add cache for isVoicemail check.

bindView() gets called many times at dialer startup and layout updates.
Besides being costly, it invokes cross-process TelecomManager methods.
This change addresses the cross-process issues by adding a short-lived
cache to isVoicemail() method invocations.

Change-Id: Ib69c0eb3969a1b7d77c9fd1a2aa6e578a31fb5e9