[Android] Rename utility method GetMainProfileKey to GetLastUsedProfileKey

Rename ::android::GetMainProfileKey to ::android::GetLastUsedProfileKey so as
to be the same as the comparable function ProfileManager::GetLastUsedProfile.

TBR=It is just a rename/refactor.

Change-Id: I78f639cd6deb90bd34e672ed782fda7373ae9b71
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1676447
Reviewed-by: Xi Han <hanxi@chromium.org>
Reviewed-by: Henrique Nakashima <hnakashima@chromium.org>
Reviewed-by: Mohamed Heikal <mheikal@chromium.org>
Commit-Queue: Mohamed Heikal <mheikal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#672228}
7 files changed