mtp: add <assert.h> and <functional> includes

Fix storageID -> mStorageID typo: This assertion is never compiled,
because NDEBUG is defined. However, this code also doesn't include
assert.h, and upgrading libc++ incidentally removed a transitive
assert.h include. With the assert macro missing, the compiler finally
compiled the info->storageID expression and diagnosed its typo.

Bug: 175635923
Test: m MODULES-IN-frameworks-av-media-mtp
Change-Id: I9e331f28722e8eba314f7f9a731c57f6ee8cb99e