MTP: Skip runtime thumbnail generation for Android Host

Android Host is not capable to deal with the long processing time for
runtime thumbnail generation.
Skip runtime thumbnail generation for Android Host
but keep no impact to other Host

Bug: 188752500
Test: atest MtpTests
Test: atest MtpServerTest
Test: atest MtpServiceTests
Test: manual test on Windows 10 File Explorer
Test: manual test on Android Host Files App
Change-Id: I24ed3bb29c5a6912398a9e00e7748fd406cf6a64