android: extract LOAbout from LibreOfficeMainActivity

Change-Id: Ied6a9a64041de94766b307c7ca64eba7edeab64c
android: show about menu entry in the browser as well

Change-Id: I74a38281dc80790f7593042c205c31a6fc2d2cdc
android: fix loading documents from doc browser's about dialog

- LibreOfficeUIActivity needs to start a new activity
- LibreOfficeMainActivity is enough to send a new event

Change-Id: I3a7532a07b37a16bdb49f81072132aa57c2c52e8