[RNMobile] Show "Add to beginning" and "Add to end" option always

[AmandaRiu]

Description

Closes wordpress-mobile/gutenberg-mobile#2344 by making the long-press options Add to beginning and Add to end available in all scenarios. Prior to this PR these options were only available when no blocks were selected.

Related PR

gutenberg-mobile: wordpress-mobile/gutenberg-mobile#3074

How has this been tested?

1. Open the demo app
2. Long-press on the inserter button, verify options Add to beginning and Add to end are displayed.
3. Tap on Add to beginning and select a block. Verify block added to the beginning.
4. With no block selected, long-press on inserter again, this time tap on Add to end and select a block. Verify added to end of post.
5. Select a block in the editor, then long-press on the inserter. Verify new options Add to beginning and Add to end are displayed.
6. Tap on Add to beginning and select a block. Verify added to beginning of document.
7. Repeat for the other options available when long-pressing on inserter with a block selected and verify each work.

Screenshots

No block selected BEFORE	AFTER

Block selected BEFORE	AFTER

Types of changes

• Updated logic to show the Add to beginning and Add to end option always.
• Changed the icon for Add block before and Add block after to plusCircle per design recommendation here.

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.
• I've updated all React Native files affected by any refactorings/renamings in this PR.

[cameronvoell]

Changes are looking good @AmandaRiu , nice work!

Figured I'd note here that I also tested inside the group block, and the add to beginning and end would stay confined to the group block which is what I expected to happen. Looks good!