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
diff --git a/media/mtp/Android.bp b/media/mtp/Android.bp
index 543ad5c..acea373 100644
--- a/media/mtp/Android.bp
+++ b/media/mtp/Android.bp
@@ -19,6 +19,7 @@
     srcs: [
         "MtpDataPacket.cpp",
         "MtpDebug.cpp",
+        "MtpDescriptors.cpp",
         "MtpDevHandle.cpp",
         "MtpDevice.cpp",
         "MtpDeviceInfo.cpp",