Fix: crash when running am.jar without parameters

Issure:
When no args to run am.jar,mAm is not initialized before use
process will crash in NRE

Solution:
Initialize mAm in constructor

Bug: 202471754
Test: manual
Running follow sh on the device displays help messages correctly

base=/system
export CLASSPATH=$base/framework/am.jar
exec app_process $base/bin com.android.commands.am.Am

Change-Id: I088f4f5b4072d350c217655a291658d0bfd506e9