Remember hex type to support MBR creation.

Before this patch, requesting a code like "0x0c00" would be flattened
to "0x0700" because it was translated through the GUID type table.

Callers creating MBR tables expect these codes to flow through without
this fidelity loss.  To support this, we stash away any provided hex
type and return it instead of doing a lookup.

Test: cts-tradefed run commandAndExit cts-dev --abi armeabi-v7a -m CtsAppSecurityHostTestCases -t android.appsecurity.cts.AdoptableHostTest
Bug: 63735902
Change-Id: If61407960c9ad1050e5f9031617d71d68f7820b4