Specifies RECEIVER_EXPORTED for FlashNotificationsController broadcast receiver.

As of Android U broadcast receivers for non-system broadcasts
must specify RECEIVER_EXPORTED or RECEIVER_NOT_EXPORTED.
See go/receiver-flag-enforcement-design.

RECEIVER_EXPORTED is the default (pre-U) behavior.
b/266775677 is evaluating whether this can be changed to
RECEIVER_NOT_EXPORTED.

Bug: 268113696
Bug: 266775677
Change-Id: Ib0334262c1f58596eaba2581a20b7595e982df0c
Test: Use go/abtd to run com.google.android.projection.gearhead.test.functional#CarAppTest-searchTemplate