Verify presence of _FileAsset::getBuffer data

Rather than attempt to remove all users of Asset::getBuffer (which
includes AssetManager(1), aapt(1), and other places) and migrate  them
to use Asset::getIncFsBuffer, verify the presence of all the data in
the buffer before returning a raw pointer to the buffer data to
guarantee callers will not unexpectedly get a SIGBUS due to incremental
installation.

Bug: 179254882
Test: builds
Change-Id: I24fd9036bc53a8c23166b5471862ee542630fb56