Unexport OmtpMessageReceiver

Introduced in
https://partner-android-review.googlesource.com/#/c/204705/

Receivers do not need to be exported to receive system broadcasts.
OmtpMessageReceiver should also check the intent action, but least
possible scope of change is preferred.

Bug: 31566390
Change-Id: I83f3d42ba185c072357ec6f18b99c3f12f452eb9