Add settings to toggle the phantom process monitoring in dev options

For power users, the monitoring on phantom processes could be turned
off from the Settings->Developer Options->Feature flags.

Bug: 210353231
Bug: 205156966
Test: Manual
Test: atest AppChildProcessTest
Test: atest CtsAppTestCases:ActivityManagerTest
Change-Id: I05f5b43e281b98676c615d350c213620d3d72b5c
Merged-In: I05f5b43e281b98676c615d350c213620d3d72b5c