front: Remove unneeded calls to load native libraries.

To remove the call to System.exit(), we have to remove most of the
java-side library loading.  The only libraries that are ok to load from
the java-side are ones that have no dependencies on SDL2, core, or
front-end libraries.

No core library functions are called directly by the java/jni functions
(they're called through the front-end instead), and the front-end
already loads the core library dynamically from native code.  So we can
remove the java call to load the core.

We can also remove the calls to all the plugin libraries, since the
core loads these dynamically from the native side.  For the time being,
we still have to load the input plugin on the java side, because we are
initializing stuff in java.  Later this will be removed.

The end goal is to load only a single native library from the java side
for all interactions with SDL/core.  This native library (to be imple-
mented) will dynamically load the ae-imports, SDL2, and front-end libr-
aries from the native side.  This will allow all core-related libraries
to be completely unloaded and reloaded every time a game activity is
launched, which will force all global/static variables to be reinitial-
ized through their static declarations.  Otherwise we'd have to use
System.exit() to achieve the same effect, which is a last-resort kind
of solution, to be avoided wherever possible in Android programming.
ae-bridge: Removed ROM header utilities, moved to pure Java code.

No need for this to happen in native code.  This isn't the expensive
part of discerning cheats for a particular ROM.  Moving it out of the
bridge also means that the only activity that actually uses the bridge
is the GameActivity (as it should be).
front: Simplify header CRC handling.

The complicated CRC check can be replaced with a simple String.format
operation. Note that the new implementation isn't identical to the old
in every way: before you could get CRC's like "0 0" from the native
side, which would then be turned into a null after the check.  Now, if
given the same file, the final CRC would be "00000000 00000000" instead
of null.  However, at the end of the day, this makes no difference to
the downstream code; it will just fail to find a cheat entry in either
case.