Shut down cleanly when memory is low.

briar-android/src/main/AndroidManifest.xml

...ain/java/org/briarproject/briar/android/BriarService.java

...rg/briarproject/briar/android/activity/BriarActivity.java

...a/org/briarproject/briar/android/logout/ExitActivity.java

...org/briarproject/briar/android/logout/HideUiActivity.java

...rg/briarproject/briar/android/logout/SignOutFragment.java

...iarproject/briar/android/navdrawer/NavDrawerActivity.java