Fix UiccController crash due to inconsistent phone count

Using TelephonyManager.getDefault() causes mContext to be null, which
makes getPhoneCount quietly fall back to default values. This makes it
return sometimes 0, sometimes 1, depending on whether the calling code
used TelephonyManager.getDefault() or TelephonyManager.from(Context).

Bug: 404627136
Flag: EXEMPT bugfix
Test: boot gcar_x86_64-trunk_staging-userdebug and observe NPEs in logcat
Change-Id: Ibf87e7cac9595d87ac593d581928b3863953a274