Fix NPE occurring while accessing the token Map in AppSmsManager

In AppSmsManager a crash was occurring when trying to loop through
and remove elements from the token Map. Instead, an iterator is
used to perform this operation safely.

Fixes: 181897608
Test: atest CtsTelephonyTestCases
Change-Id: Icf577604c669545722448a68186da94cac2e69fb