AAPT2: Clean up tests a bit

Since the latest gtest has fixed support for
explicit bool operators, remvoe AAPT_ASSERT_* and AAPT_EXPECT_*.

Also switch to use NotNull() matchers, which are more legible.

Test: make aapt2_tests
Change-Id: Idce199ca9d567d70f7aae275fee15e04bb914c9e