Is there a way to grant myself MANAGE_USERS permission while inside the adb shell?
I solved my problem with a little help from Google and XDA Forums. Though this doesn't answer the exact title question, I found an alternative way to disable OEM bloatware on Android M+ without requiring MANAGE_USERS permission or root:

pm uninstall -k --user 0 com.micromax.trendingapps



(you may use or not the -k option, which is to keep the data and cache directories)