Adding am send-trim-memory command

This patch adds a send-trim-memory command to the ActivityManager to allow
for better debugging&testing.

The command is
adb shell am send-trim-memory [--user <USER_ID>] <PROCESS> <LEVEL>
whereas LEVEL can be one of the following:
[HIDDEN|RUNNING_MODERATE|BACKGROUND|RUNNING_LOW|MODERATE|
 RUNNING_CRITICAL|COMPLETE]

Bug: 21633189
Change-Id: I7a41ce02c3c9043ffd3e5aaa791f7b7306a9de49