Fix NPE in MtpDocumentsProviderTests

Previous patch ag/2250238 broke MtpDocumentsProviderTests since
ServiceIntentSender's constructor refers the context argument which is
null in the tests.

The CL adds a test version of the constructor which does not use the
context argument in it.

Bug: 38363487
Test: MtpDocumentsProviderTests
Change-Id: I68a1d8cb6997499e5069c33f70fd0f675aaad77c
(cherry picked from commit c5949bd3)