Convert BlockManager into BlockList

[koke]

In previous efforts we tried to port the BlockManager and related components from the Gutenberg mobile repository to the Gutenberg repository, to pave the way for inner blocks on RN mobile. However that prove to be unsuccessful as the PR was getting too big to review at once.

To give more context, BlockManager is the equivalent of BlockList and it depends on many components that still lives in the gutenberg-mobile repo.
The goal here will be to make that port so that we can ultimately have BlockList be reused from the gutenberg repository in components such as InnerBlock.

This issue will serve as a reference for this task and will help monitor the progress on the different PRs.

Most of the work was already done in #789, but we're going to break it down now into smaller pieces to make it easier to review.

Suggested plan:

• 1. Move BottomSheet and Picker to @wordpress/components make them available from outside the block-editor package (GB PR#15750 and GB-Mobile PR#1008)
• 2. Port KeyboardAvoidingView, KeyboardAwareFlatList and ReadableContentView to gutenberg so we can reuse those from there (GB PR#15913 and GB-Mobile PR#1048)
• 3. Inserter ported from BlockPicker (GB PR#16114 and GB-Mobile PR#1122)
• 4. BlockMobileToolbar ported from InlineToolbar (GB PR#16177 and GB-Mobile PR#1133)
• 5. BlockToolbar ported from BlockToolbar (GB PR#16213 and GB-Mobile PR#1149)
• 6. BlockListBlock ported from BlockHolder (GB PR#16223 and GB-Mobile PR#1155)
• 7. HTMLTextInput ported from HTMLTextInput (GB PR#16226 and GB-Mobile PR#1162)
• 8. BlockList ported from BlockManager (GB PR#16239 and GB-Mobile PR#1169)
• 8b. Port MainScreen, AppContainer and App to gutenberg (GB PR#16260 and GB-Mobile PR#1171)
• 9. Create a native version of InnerBlock by adding support for MediaText (GB PR#16305)

Originally posted in #789 (comment)

cc @koke @hypest @daniloercoli @gziolo

[koke]

Removing my assignment for now. @Tug will take over this, and I'll help where I can.