Enhanced 'build_apk.py' logic & 'README.md'.

    There was an issue in 'build_apk.py': when only 'qnnsdk' and 'model_path' were provided as inputs, it incorrectly invoked the 'printmenu()' function to display a model selection menu, despite the model always being renamed to 'classification.tflite'. To address this, I adjusted the logic so that the model selection menu is only printed when neither 'model_name' nor 'model_path' is specified by the user. Specifically, I changed line 45 of 'build_apk.py' to read:
    ```
    if not args.model_name and not args.model_path:
    ```
    
    Additionally, I made several descriptive improvements to the 'README.md' file. Recognizing that newcomers primarily aim to compile and install the ImageClassification application, I included more concise and efficient instructions tailored to their needs, aiming to minimize their trial-and-error process and facilitate a smoother onboarding experience."

Signed-off-by: w <27848155+fangxingSR@users.noreply.github.com>

Replace qai-hub with AI Hub Models.
Remove the leading space in the line starting with [ README.md......

Signed-off-by: w <27848155+fangxingSR@users.noreply.github.com>

Signed-off-by: w <27848155+fangxingSR@users.noreply.github.com>