Using following command you can disable the existing lockscreen of course if it doesn't contain any security(like patterns and pin code):
KeyguardManager keyguardManager = (KeyguardManager)getSystemService(Activity.KEYGUARD_SERVICE);
KeyguardLock lock = keyguardManager.newKeyguardLock(KEYGUARD_SERVICE);
lock.disableKeyguard();
you need also to declare permission in manifest:
<uses-permission android:name="android.permission.DISABLE_KEYGUARD"/>
Than detect the user presence (clicking on Power Button):
<receiver android:name=".Receive">
<intent-filter android:enabled="true" android:exported="false">
<action android:name="android.intent.action.USER_PRESENT" />
</intent-filter>
</receiver>
and start custom lockscreen Activity