LowpanCommissioningSession: Add `static abstract` keywords

These keywords were accidentally left off of the original declaration.

Bug: b/63645461
Change-Id: I1cadcd68831377ee1d1059f645f7178b7e5c742e
Tests: Verified by being able to successfully compile
	I46667d4ea337463a35a9dd31fb00f7e130553d53
1 file changed