Switch frameworks/base/core/jni from gcc to clang.

Switch from gcc to clang and fix errors detected.

The immediate motivation is that gcc with libc++ stl does not use
'explicit' in the stl, causing issues for use of unique_ptr.
The longer term motivation is to fully move to clang.

BUG=22414716

Change-Id: If77436c1e7601f20289603862be33b85880ab4bd
6 files changed