[android] Raise minimum API level to 16

bramble-android/build.gradle

...arproject/bramble/plugin/tor/AndroidTorPluginFactory.java

...arproject/bramble/system/AndroidSecureRandomProvider.java

briar-android/build.gradle

...ava/org/briarproject/briar/android/BackgroundMonitor.java

.../org/briarproject/briar/android/BriarApplicationImpl.java

...g/briarproject/briar/android/ScreenFilterMonitorImpl.java

...riarproject/briar/android/conversation/ImageActivity.java

...g/briarproject/briar/android/keyagreement/CameraView.java

...iarproject/briar/android/reporting/BriarReportPrimer.java

.../briarproject/briar/android/view/CompositeSendButton.java