Deprecated 'adb bugreport' with flat files.

Starting on Android N, zipped bugreports contain more information than
flat-file, text bugreports. On N, calls to 'adb bugreport' would still
generate a flat-file bugreport, but with a warning.

With this change, 'adb bugreport' will generate a zipped bugreport in
the current directory, using the bugreport name provided by the
device. Similarly, calling 'adb bugreport dir' will generate a bugreport
with a device-provided name, but in such directory.

BUG: 30451114
BUG: 29448020

Change-Id: Ibc8920dd44a5f62feb15bf3fefdcb0bdbf389a90