Some fixes and improvement of the Android makefile and gitignore

[Majanto]

After trying out this tutorial, here are some changes I recommend.

Android Makefile

• _RAYLIBPATH: Hard path replaced by a relative path matching the description in the README.
• _ANDROID_APIVERSION: Can now be set with the make command.
• clear: When running the make command a 2nd time, the APK building never stopped, with its size increasing forever. Added a clear step to remove the bin folder, so building can go normally.
• _create_project_apkpackage: Updated temp APK name so we use the correct one for the rest of the process.

Gitignore

• Added *apk* and android.* to the gitignore.

[raysan5]

@Majanto Thanks for the review!