Add small improvements

[DeFuex]

• Does add android ktx to use e.g: withStyledAttributes.
• uses @jvmoverloads to hide all constructor calls and uses kotlins init {} function instead.
• enable jvm 1.8 bytecode compilation target for withStyledAttributes function.

[YvesCheung]

Thanks for your contribution！
“Uses @jvmoverloads to hide all constructor calls and uses kotlins init {} function instead.” looks good.
However, I hesitate to use AndroidX and target jvm1.8, which might increase the threshold for users.
For now, There are still some old fat project using "support library" instead of "AndroidX".
A library should be able to adapt to as many development environments as possible.

[DeFuex]

Hi, no problem, i will change it to only use the Overload anmotation then.