Move mtp descriptors into separate source, add tests

Functionfs descriptors are moved into MtpDescriptors.cpp.
Added assertions and tests to ensure that descriptors aren't
unintentionally changed, and bug numbers in comments for
future context.

Test: updated MtpFfsHandle_test passes
Bug: 67367056
Change-Id: I90eb5e48a687f366b8919621718e976e83b5b064