Update guard to use PERFETTO_BUILDFLAG
Update guard to use PERFETTO_BUILDFLAG

Fix a left over use of defined(PERFETTO_OS_ANDROID) to use
PERFETTO_BUILDFLAG instead. Also update the ifs to test
PERFETTO_ANDROID_BUILD instead of PERFETTO_OS_ANDROID, the former
seems more correct since the code is about which task runner to use
and we only want to use the Android task runner when building
with Dropbox support.

Change-Id: Ic5804dee2db0e07295f4b7c1519136c365438f60
google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto