[pm/incremental] address API review comments for LauncherApps APIs

+ change "*Progress*" to "*LoadingProgress*".
+ removed "LauncherActivityInfo.isLoading()" because it can be derived
from "LauncherActivityInfo.getLoadingProgress()".

BUG: 171804873
Test: atest LauncherAppsSingleUserTest
Change-Id: I001e8d946a13bcf43c03081e730ac07a1c3cf4bc