AssetManager2: Fix a memory leak

Caught by the static analyzer:

frameworks/base/libs/androidfw/AssetManager2.cpp:580:9: warning:
Potential leak of memory pointed to by 'new_bag'
[clang-analyzer-unix.Malloc]

Bug: 27101951
Test: mma. Memory leak warning is gone.
Change-Id: I532585d4dd376cec1abf4358f26d23f5ae3231cf