Post cancelAsUser to mHandler.

This is to fix a java.lang.SecurityException: Caller android:1010157 cannot post for pkg android in user 10
that was coming from this line.

Bug: 153016511
Test: Manual - did make, flash, ran a test app to repro the SecurityException to verify it does not.
Change-Id: Icb1bb9982a7fb4b377b0b6773c19a146c5ea8f63