Correct exported state for AutofillManagerService receiver

The AutofillManagerService receiver that listens for
CLOSE_SYSTEM_DIALOGS broadcasts was initially marked as not exported
since this broadcast should be dropped by the platform when sent from
apps. However since this broadcast is still valid to be used in tests,
receivers registering for this broadcast should be marked as exported.
This commit updates the receiver to exported to allow any tests that
send this broadcast to complete successfully.

Bug: 161145287
Test: Build
Change-Id: Ib1a11165f25894e67ba7dc7e9bea9397b542b4f9